Merge branch 'bc/object-id'

Conversion from uchar[20] to struct object_id continues.

* bc/object-id: (25 commits)
  refs/files-backend: convert static functions to object_id
  refs: convert read_raw_ref backends to struct object_id
  refs: convert peel_object to struct object_id
  refs: convert resolve_ref_unsafe to struct object_id
  worktree: convert struct worktree to object_id
  refs: convert resolve_gitlink_ref to struct object_id
  Convert remaining callers of resolve_gitlink_ref to object_id
  sha1_file: convert index_path and index_fd to struct object_id
  refs: convert reflog_expire parameter to struct object_id
  refs: convert read_ref_at to struct object_id
  refs: convert peel_ref to struct object_id
  builtin/pack-objects: convert to struct object_id
  pack-bitmap: convert traverse_bitmap_commit_list to object_id
  refs: convert dwim_log to struct object_id
  builtin/reflog: convert remaining unsigned char uses to object_id
  refs: convert dwim_ref and expand_ref to struct object_id
  refs: convert read_ref and read_ref_full to object_id
  refs: convert resolve_refdup and refs_resolve_refdup to struct object_id
  Convert check_connected to use struct object_id
  refs: update ref transactions to use struct object_id
  ...

70 files changed, 544 insertions, 554 deletions

diff --git a/archive.c b/archive.c
index 1e41f4bbeb..0b7b62af0c 100644
--- a/archive.c
+++ b/archive.c
@@ -371,7 +371,7 @@ static void parse_treeish_arg(const char **argv,
 		const char *colon = strchrnul(name, ':');
 		int refnamelen = colon - name;
 
-		if (!dwim_ref(name, refnamelen, oid.hash, &ref))
+		if (!dwim_ref(name, refnamelen, &oid, &ref))
 			die("no such ref: %.*s", refnamelen, name);
 		free(ref);
 	}
diff --git a/bisect.c b/bisect.c
index a852cc9d19..fda2c4a186 100644
--- a/bisect.c
+++ b/bisect.c
@@ -690,11 +690,12 @@ static int bisect_checkout(const struct object_id *bisect_rev, int no_checkout)
 	char bisect_rev_hex[GIT_MAX_HEXSZ + 1];
 
 	memcpy(bisect_rev_hex, oid_to_hex(bisect_rev), GIT_SHA1_HEXSZ + 1);
-	update_ref(NULL, "BISECT_EXPECTED_REV", bisect_rev->hash, NULL, 0, UPDATE_REFS_DIE_ON_ERR);
+	update_ref(NULL, "BISECT_EXPECTED_REV", bisect_rev, NULL, 0, UPDATE_REFS_DIE_ON_ERR);
 
 	argv_checkout[2] = bisect_rev_hex;
 	if (no_checkout) {
-		update_ref(NULL, "BISECT_HEAD", bisect_rev->hash, NULL, 0, UPDATE_REFS_DIE_ON_ERR);
+		update_ref(NULL, "BISECT_HEAD", bisect_rev, NULL, 0,
+			   UPDATE_REFS_DIE_ON_ERR);
 	} else {
 		int res;
 		res = run_command_v_opt(argv_checkout, RUN_GIT_CMD);
diff --git a/blame.c b/blame.c
index f575e9cbf4..c3060de2f8 100644
--- a/blame.c
+++ b/blame.c
@@ -166,7 +166,7 @@ static struct commit *fake_working_tree_commit(struct diff_options *opt,
 	commit->date = now;
 	parent_tail = &commit->parents;
 
-	if (!resolve_ref_unsafe("HEAD", RESOLVE_REF_READING, head_oid.hash, NULL))
+	if (!resolve_ref_unsafe("HEAD", RESOLVE_REF_READING, &head_oid, NULL))
 		die("no such ref: HEAD");
 
 	parent_tail = append_parent(parent_tail, &head_oid);
@@ -1689,7 +1689,7 @@ static struct commit *dwim_reverse_initial(struct rev_info *revs,
 		return NULL;
 
 	/* Do we have HEAD? */
-	if (!resolve_ref_unsafe("HEAD", RESOLVE_REF_READING, head_oid.hash, NULL))
+	if (!resolve_ref_unsafe("HEAD", RESOLVE_REF_READING, &head_oid, NULL))
 		return NULL;
 	head_commit = lookup_commit_reference_gently(&head_oid, 1);
 	if (!head_commit)
diff --git a/branch.c b/branch.c
index 4377ce2fb1..62f7b0d8c2 100644
--- a/branch.c
+++ b/branch.c
@@ -264,7 +264,7 @@ void create_branch(const char *name, const char *start_name,
 		die(_("Not a valid object name: '%s'."), start_name);
 	}
 
-	switch (dwim_ref(start_name, strlen(start_name), oid.hash, &real_ref)) {
+	switch (dwim_ref(start_name, strlen(start_name), &oid, &real_ref)) {
 	case 0:
 		/* Not branching from any existing branch */
 		if (explicit_tracking)
@@ -305,7 +305,7 @@ void create_branch(const char *name, const char *start_name,
 		transaction = ref_transaction_begin(&err);
 		if (!transaction ||
 		    ref_transaction_update(transaction, ref.buf,
-					   oid.hash, forcing ? NULL : null_sha1,
+					   &oid, forcing ? NULL : &null_oid,
 					   0, msg, &err) ||
 		    ref_transaction_commit(transaction, &err))
 			die("%s", err.buf);
diff --git a/builtin/am.c b/builtin/am.c
index 40968428dd..4b6f1534f8 100644
--- a/builtin/am.c
+++ b/builtin/am.c
@@ -1068,8 +1068,8 @@ static void am_setup(struct am_state *state, enum patch_format patch_format,
 	if (!get_oid("HEAD", &curr_head)) {
 		write_state_text(state, "abort-safety", oid_to_hex(&curr_head));
 		if (!state->rebasing)
-			update_ref_oid("am", "ORIG_HEAD", &curr_head, NULL, 0,
-					UPDATE_REFS_DIE_ON_ERR);
+			update_ref("am", "ORIG_HEAD", &curr_head, NULL, 0,
+				   UPDATE_REFS_DIE_ON_ERR);
 	} else {
 		write_state_text(state, "abort-safety", "");
 		if (!state->rebasing)
@@ -1685,8 +1685,8 @@ static void do_commit(const struct am_state *state)
 	strbuf_addf(&sb, "%s: %.*s", reflog_msg, linelen(state->msg),
 			state->msg);
 
-	update_ref_oid(sb.buf, "HEAD", &commit, old_oid, 0,
-			UPDATE_REFS_DIE_ON_ERR);
+	update_ref(sb.buf, "HEAD", &commit, old_oid, 0,
+		   UPDATE_REFS_DIE_ON_ERR);
 
 	if (state->rebasing) {
 		FILE *fp = xfopen(am_path(state, "rewritten"), "a");
@@ -2132,7 +2132,7 @@ static void am_abort(struct am_state *state)
 
 	am_rerere_clear();
 
-	curr_branch = resolve_refdup("HEAD", 0, curr_head.hash, NULL);
+	curr_branch = resolve_refdup("HEAD", 0, &curr_head, NULL);
 	has_curr_head = curr_branch && !is_null_oid(&curr_head);
 	if (!has_curr_head)
 		hashcpy(curr_head.hash, EMPTY_TREE_SHA1_BIN);
@@ -2144,9 +2144,9 @@ static void am_abort(struct am_state *state)
 	clean_index(&curr_head, &orig_head);
 
 	if (has_orig_head)
-		update_ref_oid("am --abort", "HEAD", &orig_head,
-				has_curr_head ? &curr_head : NULL, 0,
-				UPDATE_REFS_DIE_ON_ERR);
+		update_ref("am --abort", "HEAD", &orig_head,
+			   has_curr_head ? &curr_head : NULL, 0,
+			   UPDATE_REFS_DIE_ON_ERR);
 	else if (curr_branch)
 		delete_ref(NULL, curr_branch, NULL, REF_NODEREF);
 
diff --git a/builtin/branch.c b/builtin/branch.c
index 79dc9181fd..b1ed649300 100644
--- a/builtin/branch.c
+++ b/builtin/branch.c
@@ -125,7 +125,7 @@ static int branch_merged(int kind, const char *name,
 		if (upstream &&
 		    (reference_name = reference_name_to_free =
 		     resolve_refdup(upstream, RESOLVE_REF_READING,
-				    oid.hash, NULL)) != NULL)
+				    &oid, NULL)) != NULL)
 			reference_rev = lookup_commit_reference(&oid);
 	}
 	if (!reference_rev)
@@ -241,7 +241,7 @@ static int delete_branches(int argc, const char **argv, int force, int kinds,
 					RESOLVE_REF_READING
 					| RESOLVE_REF_NO_RECURSE
 					| RESOLVE_REF_ALLOW_BAD_NAME,
-					oid.hash, &flags);
+					&oid, &flags);
 		if (!target) {
 			error(remote_branch
 			      ? _("remote-tracking branch '%s' not found.")
@@ -257,7 +257,7 @@ static int delete_branches(int argc, const char **argv, int force, int kinds,
 			goto next;
 		}
 
-		if (delete_ref(NULL, name, is_null_oid(&oid) ? NULL : oid.hash,
+		if (delete_ref(NULL, name, is_null_oid(&oid) ? NULL : &oid,
 			       REF_NODEREF)) {
 			error(remote_branch
 			      ? _("Error deleting remote-tracking branch '%s'")
@@ -636,7 +636,7 @@ int cmd_branch(int argc, const char **argv, const char *prefix)
 
 	track = git_branch_track;
 
-	head = resolve_refdup("HEAD", 0, head_oid.hash, NULL);
+	head = resolve_refdup("HEAD", 0, &head_oid, NULL);
 	if (!head)
 		die(_("Failed to resolve HEAD as a valid ref."));
 	if (!strcmp(head, "HEAD"))
diff --git a/builtin/checkout.c b/builtin/checkout.c
index fcff0de198..6c2b4cd419 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -377,7 +377,7 @@ static int checkout_paths(const struct checkout_opts *opts,
 	if (write_locked_index(&the_index, &lock_file, COMMIT_LOCK))
 		die(_("unable to write new index file"));
 
-	read_ref_full("HEAD", 0, rev.hash, NULL);
+	read_ref_full("HEAD", 0, &rev, NULL);
 	head = lookup_commit_reference_gently(&rev, 1);
 
 	errs |= post_checkout_hook(head, head, 0);
@@ -662,7 +662,7 @@ static void update_refs_for_switch(const struct checkout_opts *opts,
 	if (!strcmp(new->name, "HEAD") && !new->path && !opts->force_detach) {
 		/* Nothing to do. */
 	} else if (opts->force_detach || !new->path) {	/* No longer on any branch. */
-		update_ref(msg.buf, "HEAD", new->commit->object.oid.hash, NULL,
+		update_ref(msg.buf, "HEAD", &new->commit->object.oid, NULL,
 			   REF_NODEREF, UPDATE_REFS_DIE_ON_ERR);
 		if (!opts->quiet) {
 			if (old->path &&
@@ -825,7 +825,7 @@ static int switch_branches(const struct checkout_opts *opts,
 	struct object_id rev;
 	int flag, writeout_error = 0;
 	memset(&old, 0, sizeof(old));
-	old.path = path_to_free = resolve_refdup("HEAD", 0, rev.hash, &flag);
+	old.path = path_to_free = resolve_refdup("HEAD", 0, &rev, &flag);
 	if (old.path)
 		old.commit = lookup_commit_reference_gently(&rev, 1);
 	if (!(flag & REF_ISSYMREF))
@@ -1036,7 +1036,7 @@ static int parse_branchname_arg(int argc, const char **argv,
 	setup_branch_path(new);
 
 	if (!check_refname_format(new->path, 0) &&
-	    !read_ref(new->path, branch_rev.hash))
+	    !read_ref(new->path, &branch_rev))
 		oidcpy(rev, &branch_rev);
 	else
 		new->path = NULL; /* not an existing branch */
@@ -1134,7 +1134,7 @@ static int checkout_branch(struct checkout_opts *opts,
 		struct object_id rev;
 		int flag;
 
-		if (!read_ref_full("HEAD", 0, rev.hash, &flag) &&
+		if (!read_ref_full("HEAD", 0, &rev, &flag) &&
 		    (flag & REF_ISSYMREF) && is_null_oid(&rev))
 			return switch_unborn_to_new_branch(opts);
 	}
diff --git a/builtin/clone.c b/builtin/clone.c
index 96a3aaaa1f..cf6eddc9c5 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -588,7 +588,7 @@ static void write_remote_refs(const struct ref *local_refs)
 	for (r = local_refs; r; r = r->next) {
 		if (!r->peer_ref)
 			continue;
-		if (ref_transaction_create(t, r->peer_ref->name, r->old_oid.hash,
+		if (ref_transaction_create(t, r->peer_ref->name, &r->old_oid,
 					   0, NULL, &err))
 			die("%s", err.buf);
 	}
@@ -610,12 +610,12 @@ static void write_followtags(const struct ref *refs, const char *msg)
 			continue;
 		if (!has_object_file(&ref->old_oid))
 			continue;
-		update_ref(msg, ref->name, ref->old_oid.hash,
-			   NULL, 0, UPDATE_REFS_DIE_ON_ERR);
+		update_ref(msg, ref->name, &ref->old_oid, NULL, 0,
+			   UPDATE_REFS_DIE_ON_ERR);
 	}
 }
 
-static int iterate_ref_map(void *cb_data, unsigned char sha1[20])
+static int iterate_ref_map(void *cb_data, struct object_id *oid)
 {
 	struct ref **rm = cb_data;
 	struct ref *ref = *rm;
@@ -630,7 +630,7 @@ static int iterate_ref_map(void *cb_data, unsigned char sha1[20])
 	if (!ref)
 		return -1;
 
-	hashcpy(sha1, ref->old_oid.hash);
+	oidcpy(oid, &ref->old_oid);
 	*rm = ref->next;
 	return 0;
 }
@@ -682,23 +682,23 @@ static void update_head(const struct ref *our, const struct ref *remote,
 		if (create_symref("HEAD", our->name, NULL) < 0)
 			die(_("unable to update HEAD"));
 		if (!option_bare) {
-			update_ref(msg, "HEAD", our->old_oid.hash, NULL, 0,
+			update_ref(msg, "HEAD", &our->old_oid, NULL, 0,
 				   UPDATE_REFS_DIE_ON_ERR);
 			install_branch_config(0, head, option_origin, our->name);
 		}
 	} else if (our) {
 		struct commit *c = lookup_commit_reference(&our->old_oid);
 		/* --branch specifies a non-branch (i.e. tags), detach HEAD */
-		update_ref(msg, "HEAD", c->object.oid.hash,
-			   NULL, REF_NODEREF, UPDATE_REFS_DIE_ON_ERR);
+		update_ref(msg, "HEAD", &c->object.oid, NULL, REF_NODEREF,
+			   UPDATE_REFS_DIE_ON_ERR);
 	} else if (remote) {
 		/*
 		 * We know remote HEAD points to a non-branch, or
 		 * HEAD points to a branch but we don't know which one.
 		 * Detach HEAD in all these cases.
 		 */
-		update_ref(msg, "HEAD", remote->old_oid.hash,
-			   NULL, REF_NODEREF, UPDATE_REFS_DIE_ON_ERR);
+		update_ref(msg, "HEAD", &remote->old_oid, NULL, REF_NODEREF,
+			   UPDATE_REFS_DIE_ON_ERR);
 	}
 }
 
@@ -715,7 +715,7 @@ static int checkout(int submodule_progress)
 	if (option_no_checkout)
 		return 0;
 
-	head = resolve_refdup("HEAD", RESOLVE_REF_READING, oid.hash, NULL);
+	head = resolve_refdup("HEAD", RESOLVE_REF_READING, &oid, NULL);
 	if (!head) {
 		warning(_("remote HEAD refers to nonexistent ref, "
 			  "unable to checkout.\n"));
diff --git a/builtin/commit.c b/builtin/commit.c
index 1a4ad725bc..c38542ee46 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1790,9 +1790,9 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 
 	transaction = ref_transaction_begin(&err);
 	if (!transaction ||
-	    ref_transaction_update(transaction, "HEAD", oid.hash,
+	    ref_transaction_update(transaction, "HEAD", &oid,
 				   current_head
-				   ? current_head->object.oid.hash : null_sha1,
+				   ? &current_head->object.oid : &null_oid,
 				   0, sb.buf, &err) ||
 	    ref_transaction_commit(transaction, &err)) {
 		rollback_index_files();
diff --git a/builtin/describe.c b/builtin/describe.c
index 979556d9fb..e14e162ef6 100644
--- a/builtin/describe.c
+++ b/builtin/describe.c
@@ -181,7 +181,7 @@ static int get_name(const char *path, const struct object_id *oid, int flag, voi
 	}
 
 	/* Is it annotated? */
-	if (!peel_ref(path, peeled.hash)) {
+	if (!peel_ref(path, &peeled)) {
 		is_annotated = !!oidcmp(oid, &peeled);
 	} else {
 		oidcpy(&peeled, oid);
diff --git a/builtin/fast-export.c b/builtin/fast-export.c
index 2fb60d6d48..d74c73f777 100644
--- a/builtin/fast-export.c
+++ b/builtin/fast-export.c
@@ -823,7 +823,7 @@ static void get_tags_and_duplicates(struct rev_cmdline_info *info)
 		if (e->flags & UNINTERESTING)
 			continue;
 
-		if (dwim_ref(e->name, strlen(e->name), oid.hash, &full_name) != 1)
+		if (dwim_ref(e->name, strlen(e->name), &oid, &full_name) != 1)
 			continue;
 
 		if (refspecs) {
diff --git a/builtin/fetch.c b/builtin/fetch.c
index 225c734924..e705237fa9 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -457,8 +457,8 @@ static int s_update_ref(const char *action,
 	transaction = ref_transaction_begin(&err);
 	if (!transaction ||
 	    ref_transaction_update(transaction, ref->name,
-				   ref->new_oid.hash,
-				   check_old ? ref->old_oid.hash : NULL,
+				   &ref->new_oid,
+				   check_old ? &ref->old_oid : NULL,
 				   0, msg, &err))
 		goto fail;
 
@@ -727,7 +727,7 @@ static int update_local_ref(struct ref *ref,
 	}
 }
 
-static int iterate_ref_map(void *cb_data, unsigned char sha1[20])
+static int iterate_ref_map(void *cb_data, struct object_id *oid)
 {
 	struct ref **rm = cb_data;
 	struct ref *ref = *rm;
@@ -737,7 +737,7 @@ static int iterate_ref_map(void *cb_data, unsigned char sha1[20])
 	if (!ref)
 		return -1; /* end of the list */
 	*rm = ref->next;
-	hashcpy(sha1, ref->old_oid.hash);
+	oidcpy(oid, &ref->old_oid);
 	return 0;
 }
 
diff --git a/builtin/fmt-merge-msg.c b/builtin/fmt-merge-msg.c
index e99b5ddbf9..b69f7d3be2 100644
--- a/builtin/fmt-merge-msg.c
+++ b/builtin/fmt-merge-msg.c
@@ -603,7 +603,7 @@ int fmt_merge_msg(struct strbuf *in, struct strbuf *out,
 
 	/* get current branch */
 	current_branch = current_branch_to_free =
-		resolve_refdup("HEAD", RESOLVE_REF_READING, head_oid.hash, NULL);
+		resolve_refdup("HEAD", RESOLVE_REF_READING, &head_oid, NULL);
 	if (!current_branch)
 		die("No current branch");
 	if (starts_with(current_branch, "refs/heads/"))
diff --git a/builtin/fsck.c b/builtin/fsck.c
index 56afe405b8..5f91116d73 100644
--- a/builtin/fsck.c
+++ b/builtin/fsck.c
@@ -555,7 +555,7 @@ static int fsck_head_link(void)
 	if (verbose)
 		fprintf(stderr, "Checking HEAD link\n");
 
-	head_points_at = resolve_ref_unsafe("HEAD", 0, head_oid.hash, NULL);
+	head_points_at = resolve_ref_unsafe("HEAD", 0, &head_oid, NULL);
 	if (!head_points_at) {
 		errors_found |= ERROR_REFS;
 		return error("Invalid HEAD");
diff --git a/builtin/log.c b/builtin/log.c
index d81a09051e..ba9d4cd786 100644
--- a/builtin/log.c
+++ b/builtin/log.c
@@ -975,7 +975,7 @@ static char *find_branch_name(struct rev_info *rev)
 		return NULL;
 	ref = rev->cmdline.rev[positive].name;
 	tip_oid = &rev->cmdline.rev[positive].item->oid;
-	if (dwim_ref(ref, strlen(ref), branch_oid.hash, &full_ref) &&
+	if (dwim_ref(ref, strlen(ref), &branch_oid, &full_ref) &&
 	    skip_prefix(full_ref, "refs/heads/", &v) &&
 	    !oidcmp(tip_oid, &branch_oid))
 		branch = xstrdup(v);
diff --git a/builtin/merge-base.c b/builtin/merge-base.c
index 6dbd167d3b..e99f5405ce 100644
--- a/builtin/merge-base.c
+++ b/builtin/merge-base.c
@@ -156,7 +156,7 @@ static int handle_fork_point(int argc, const char **argv)
 	struct commit_list *bases;
 	int i, ret = 0;
 
-	switch (dwim_ref(argv[0], strlen(argv[0]), oid.hash, &refname)) {
+	switch (dwim_ref(argv[0], strlen(argv[0]), &oid, &refname)) {
 	case 0:
 		die("No such ref: '%s'", argv[0]);
 	case 1:
diff --git a/builtin/merge.c b/builtin/merge.c
index ab5ffe85e8..6071dbfe34 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -405,9 +405,8 @@ static void finish(struct commit *head_commit,
 			printf(_("No merge message -- not updating HEAD\n"));
 		else {
 			const char *argv_gc_auto[] = { "gc", "--auto", NULL };
-			update_ref(reflog_message.buf, "HEAD",
-				new_head->hash, head->hash, 0,
-				UPDATE_REFS_DIE_ON_ERR);
+			update_ref(reflog_message.buf, "HEAD", new_head, head,
+				   0, UPDATE_REFS_DIE_ON_ERR);
 			/*
 			 * We ignore errors in 'gc --auto', since the
 			 * user should see them.
@@ -455,7 +454,7 @@ static void merge_name(const char *remote, struct strbuf *msg)
 	if (!remote_head)
 		die(_("'%s' does not point to a commit"), remote);
 
-	if (dwim_ref(remote, strlen(remote), branch_head.hash, &found_ref) > 0) {
+	if (dwim_ref(remote, strlen(remote), &branch_head, &found_ref) > 0) {
 		if (starts_with(found_ref, "refs/heads/")) {
 			strbuf_addf(msg, "%s\t\tbranch '%s' of .\n",
 				    oid_to_hex(&branch_head), remote);
@@ -1143,7 +1142,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	 * Check if we are _not_ on a detached HEAD, i.e. if there is a
 	 * current branch.
 	 */
-	branch = branch_to_free = resolve_refdup("HEAD", 0, head_oid.hash, NULL);
+	branch = branch_to_free = resolve_refdup("HEAD", 0, &head_oid, NULL);
 	if (branch)
 		skip_prefix(branch, "refs/heads/", &branch);
 	if (!branch || is_null_oid(&head_oid))
@@ -1261,8 +1260,8 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 			die(_("Can merge only exactly one commit into empty head"));
 		remote_head_oid = &remoteheads->item->object.oid;
 		read_empty(remote_head_oid->hash, 0);
-		update_ref("initial pull", "HEAD", remote_head_oid->hash,
-			   NULL, 0, UPDATE_REFS_DIE_ON_ERR);
+		update_ref("initial pull", "HEAD", remote_head_oid, NULL, 0,
+			   UPDATE_REFS_DIE_ON_ERR);
 		goto done;
 	}
 
@@ -1357,8 +1356,8 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		free(list);
 	}
 
-	update_ref("updating ORIG_HEAD", "ORIG_HEAD", head_commit->object.oid.hash,
-		   NULL, 0, UPDATE_REFS_DIE_ON_ERR);
+	update_ref("updating ORIG_HEAD", "ORIG_HEAD",
+		   &head_commit->object.oid, NULL, 0, UPDATE_REFS_DIE_ON_ERR);
 
 	if (remoteheads && !common) {
 		/* No common ancestors found. */
diff --git a/builtin/notes.c b/builtin/notes.c
index 8e54f2d146..12afdf1907 100644
--- a/builtin/notes.c
+++ b/builtin/notes.c
@@ -724,7 +724,7 @@ static int merge_commit(struct notes_merge_options *o)
 	init_notes(t, "NOTES_MERGE_PARTIAL", combine_notes_overwrite, 0);
 
 	o->local_ref = local_ref_to_free =
-		resolve_refdup("NOTES_MERGE_REF", 0, oid.hash, NULL);
+		resolve_refdup("NOTES_MERGE_REF", 0, &oid, NULL);
 	if (!o->local_ref)
 		die(_("failed to resolve NOTES_MERGE_REF"));
 
@@ -736,8 +736,8 @@ static int merge_commit(struct notes_merge_options *o)
 	format_commit_message(partial, "%s", &msg, &pretty_ctx);
 	strbuf_trim(&msg);
 	strbuf_insert(&msg, 0, "notes: ", 7);
-	update_ref(msg.buf, o->local_ref, oid.hash,
-		   is_null_oid(&parent_oid) ? NULL : parent_oid.hash,
+	update_ref(msg.buf, o->local_ref, &oid,
+		   is_null_oid(&parent_oid) ? NULL : &parent_oid,
 		   0, UPDATE_REFS_DIE_ON_ERR);
 
 	free_notes(t);
@@ -850,12 +850,12 @@ static int merge(int argc, const char **argv, const char *prefix)
 
 	if (result >= 0) /* Merge resulted (trivially) in result_oid */
 		/* Update default notes ref with new commit */
-		update_ref(msg.buf, default_notes_ref(), result_oid.hash, NULL,
-			   0, UPDATE_REFS_DIE_ON_ERR);
+		update_ref(msg.buf, default_notes_ref(), &result_oid, NULL, 0,
+			   UPDATE_REFS_DIE_ON_ERR);
 	else { /* Merge has unresolved conflicts */
 		const struct worktree *wt;
 		/* Update .git/NOTES_MERGE_PARTIAL with partial merge result */
-		update_ref(msg.buf, "NOTES_MERGE_PARTIAL", result_oid.hash, NULL,
+		update_ref(msg.buf, "NOTES_MERGE_PARTIAL", &result_oid, NULL,
 			   0, UPDATE_REFS_DIE_ON_ERR);
 		/* Store ref-to-be-updated into .git/NOTES_MERGE_REF */
 		wt = find_shared_symref("NOTES_MERGE_REF", default_notes_ref());
diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 6e77dfd444..631de28761 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -151,7 +151,7 @@ static unsigned long do_compress(void **pptr, unsigned long size)
 }
 
 static unsigned long write_large_blob_data(struct git_istream *st, struct sha1file *f,
-					   const unsigned char *sha1)
+					   const struct object_id *oid)
 {
 	git_zstream stream;
 	unsigned char ibuf[1024 * 16];
@@ -165,7 +165,7 @@ static unsigned long write_large_blob_data(struct git_istream *st, struct sha1fi
 		int zret = Z_OK;
 		readlen = read_istream(st, ibuf, sizeof(ibuf));
 		if (readlen == -1)
-			die(_("unable to read %s"), sha1_to_hex(sha1));
+			die(_("unable to read %s"), oid_to_hex(oid));
 
 		stream.next_in = ibuf;
 		stream.avail_in = readlen;
@@ -339,7 +339,7 @@ static unsigned long write_no_reuse_object(struct sha1file *f, struct object_ent
 		sha1write(f, header, hdrlen);
 	}
 	if (st) {
-		datalen = write_large_blob_data(st, f, entry->idx.oid.hash);
+		datalen = write_large_blob_data(st, f, &entry->idx.oid);
 		close_istream(st);
 	} else {
 		sha1write(f, buf, datalen);
@@ -557,13 +557,13 @@ static enum write_one_status write_one(struct sha1file *f,
 static int mark_tagged(const char *path, const struct object_id *oid, int flag,
 		       void *cb_data)
 {
-	unsigned char peeled[20];
+	struct object_id peeled;
 	struct object_entry *entry = packlist_find(&to_pack, oid->hash, NULL);
 
 	if (entry)
 		entry->tagged = 1;
-	if (!peel_ref(path, peeled)) {
-		entry = packlist_find(&to_pack, peeled, NULL);
+	if (!peel_ref(path, &peeled)) {
+		entry = packlist_find(&to_pack, peeled.hash, NULL);
 		if (entry)
 			entry->tagged = 1;
 	}
@@ -792,7 +792,7 @@ static void write_pack_file(void)
 	write_order = compute_write_order();
 
 	do {
-		unsigned char sha1[20];
+		struct object_id oid;
 		char *pack_tmp_name = NULL;
 
 		if (pack_to_stdout)
@@ -823,13 +823,13 @@ static void write_pack_file(void)
 		 * If so, rewrite it like in fast-import
 		 */
 		if (pack_to_stdout) {
-			sha1close(f, sha1, CSUM_CLOSE);
+			sha1close(f, oid.hash, CSUM_CLOSE);
 		} else if (nr_written == nr_remaining) {
-			sha1close(f, sha1, CSUM_FSYNC);
+			sha1close(f, oid.hash, CSUM_FSYNC);
 		} else {
-			int fd = sha1close(f, sha1, 0);
-			fixup_pack_header_footer(fd, sha1, pack_tmp_name,
-						 nr_written, sha1, offset);
+			int fd = sha1close(f, oid.hash, 0);
+			fixup_pack_header_footer(fd, oid.hash, pack_tmp_name,
+						 nr_written, oid.hash, offset);
 			close(fd);
 			if (write_bitmap_index) {
 				warning(_(no_split_warning));
@@ -863,16 +863,16 @@ static void write_pack_file(void)
 			strbuf_addf(&tmpname, "%s-", base_name);
 
 			if (write_bitmap_index) {
-				bitmap_writer_set_checksum(sha1);
+				bitmap_writer_set_checksum(oid.hash);
 				bitmap_writer_build_type_index(written_list, nr_written);
 			}
 
 			finish_tmp_packfile(&tmpname, pack_tmp_name,
 					    written_list, nr_written,
-					    &pack_idx_opts, sha1);
+					    &pack_idx_opts, oid.hash);
 
 			if (write_bitmap_index) {
-				strbuf_addf(&tmpname, "%s.bitmap", sha1_to_hex(sha1));
+				strbuf_addf(&tmpname, "%s.bitmap", oid_to_hex(&oid));
 
 				stop_progress(&progress_state);
 
@@ -887,7 +887,7 @@ static void write_pack_file(void)
 
 			strbuf_release(&tmpname);
 			free(pack_tmp_name);
-			puts(sha1_to_hex(sha1));
+			puts(oid_to_hex(&oid));
 		}
 
 		/* mark written objects as written to previous pack */
@@ -928,13 +928,13 @@ static int no_try_delta(const char *path)
  * found the item, since that saves us from having to look it up again a
  * few lines later when we want to add the new entry.
  */
-static int have_duplicate_entry(const unsigned char *sha1,
+static int have_duplicate_entry(const struct object_id *oid,
 				int exclude,
 				uint32_t *index_pos)
 {
 	struct object_entry *entry;
 
-	entry = packlist_find(&to_pack, sha1, index_pos);
+	entry = packlist_find(&to_pack, oid->hash, index_pos);
 	if (!entry)
 		return 0;
 
@@ -990,7 +990,7 @@ static int want_found_object(int exclude, struct packed_git *p)
  * function finds if there is any pack that has the object and returns the pack
  * and its offset in these variables.
  */
-static int want_object_in_pack(const unsigned char *sha1,
+static int want_object_in_pack(const struct object_id *oid,
 			       int exclude,
 			       struct packed_git **found_pack,
 			       off_t *found_offset)
@@ -998,7 +998,7 @@ static int want_object_in_pack(const unsigned char *sha1,
 	struct mru_entry *entry;
 	int want;
 
-	if (!exclude && local && has_loose_object_nonlocal(sha1))
+	if (!exclude && local && has_loose_object_nonlocal(oid->hash))
 		return 0;
 
 	/*
@@ -1019,7 +1019,7 @@ static int want_object_in_pack(const unsigned char *sha1,
 		if (p == *found_pack)
 			offset = *found_offset;
 		else
-			offset = find_pack_entry_one(sha1, p);
+			offset = find_pack_entry_one(oid->hash, p);
 
 		if (offset) {
 			if (!*found_pack) {
@@ -1039,7 +1039,7 @@ static int want_object_in_pack(const unsigned char *sha1,
 	return 1;
 }
 
-static void create_object_entry(const unsigned char *sha1,
+static void create_object_entry(const struct object_id *oid,
 				enum object_type type,
 				uint32_t hash,
 				int exclude,
@@ -1050,7 +1050,7 @@ static void create_object_entry(const unsigned char *sha1,
 {
 	struct object_entry *entry;
 
-	entry = packlist_alloc(&to_pack, sha1, index_pos);
+	entry = packlist_alloc(&to_pack, oid->hash, index_pos);
 	entry->hash = hash;
 	if (type)
 		entry->type = type;
@@ -1070,17 +1070,17 @@ static const char no_closure_warning[] = N_(
 "disabling bitmap writing, as some objects are not being packed"
 );
 
-static int add_object_entry(const unsigned char *sha1, enum object_type type,
+static int add_object_entry(const struct object_id *oid, enum object_type type,
 			    const char *name, int exclude)
 {
 	struct packed_git *found_pack = NULL;
 	off_t found_offset = 0;
 	uint32_t index_pos;
 
-	if (have_duplicate_entry(sha1, exclude, &index_pos))
+	if (have_duplicate_entry(oid, exclude, &index_pos))
 		return 0;
 
-	if (!want_object_in_pack(sha1, exclude, &found_pack, &found_offset)) {
+	if (!want_object_in_pack(oid, exclude, &found_pack, &found_offset)) {
 		/* The pack is missing an object, so it will not have closure */
 		if (write_bitmap_index) {
 			warning(_(no_closure_warning));
@@ -1089,7 +1089,7 @@ static int add_object_entry(const unsigned char *sha1, enum object_type type,
 		return 0;
 	}
 
-	create_object_entry(sha1, type, pack_name_hash(name),
+	create_object_entry(oid, type, pack_name_hash(name),
 			    exclude, name && no_try_delta(name),
 			    index_pos, found_pack, found_offset);
 
@@ -1097,27 +1097,27 @@ static int add_object_entry(const unsigned char *sha1, enum object_type type,
 	return 1;
 }
 
-static int add_object_entry_from_bitmap(const unsigned char *sha1,
+static int add_object_entry_from_bitmap(const struct object_id *oid,
 					enum object_type type,
 					int flags, uint32_t name_hash,
 					struct packed_git *pack, off_t offset)
 {
 	uint32_t index_pos;
 
-	if (have_duplicate_entry(sha1, 0, &index_pos))
+	if (have_duplicate_entry(oid, 0, &index_pos))
 		return 0;
 
-	if (!want_object_in_pack(sha1, 0, &pack, &offset))
+	if (!want_object_in_pack(oid, 0, &pack, &offset))
 		return 0;
 
-	create_object_entry(sha1, type, name_hash, 0, 0, index_pos, pack, offset);
+	create_object_entry(oid, type, name_hash, 0, 0, index_pos, pack, offset);
 
 	display_progress(progress_state, nr_result);
 	return 1;
 }
 
 struct pbase_tree_cache {
-	unsigned char sha1[20];
+	struct object_id oid;
 	int ref;
 	int temporary;
 	void *tree_data;
@@ -1125,9 +1125,9 @@ struct pbase_tree_cache {
 };
 
 static struct pbase_tree_cache *(pbase_tree_cache[256]);
-static int pbase_tree_cache_ix(const unsigned char *sha1)
+static int pbase_tree_cache_ix(const struct object_id *oid)
 {
-	return sha1[0] % ARRAY_SIZE(pbase_tree_cache);
+	return oid->hash[0] % ARRAY_SIZE(pbase_tree_cache);
 }
 static int pbase_tree_cache_ix_incr(int ix)
 {
@@ -1144,14 +1144,14 @@ static struct pbase_tree {
 	struct pbase_tree_cache pcache;
 } *pbase_tree;
 
-static struct pbase_tree_cache *pbase_tree_get(const unsigned char *sha1)
+static struct pbase_tree_cache *pbase_tree_get(const struct object_id *oid)
 {
 	struct pbase_tree_cache *ent, *nent;
 	void *data;
 	unsigned long size;
 	enum object_type type;
 	int neigh;
-	int my_ix = pbase_tree_cache_ix(sha1);
+	int my_ix = pbase_tree_cache_ix(oid);
 	int available_ix = -1;
 
 	/* pbase-tree-cache acts as a limited hashtable.
@@ -1160,7 +1160,7 @@ static struct pbase_tree_cache *pbase_tree_get(const unsigned char *sha1)
 	 */
 	for (neigh = 0; neigh < 8; neigh++) {
 		ent = pbase_tree_cache[my_ix];
-		if (ent && !hashcmp(ent->sha1, sha1)) {
+		if (ent && !oidcmp(&ent->oid, oid)) {
 			ent->ref++;
 			return ent;
 		}
@@ -1176,7 +1176,7 @@ static struct pbase_tree_cache *pbase_tree_get(const unsigned char *sha1)
 	/* Did not find one.  Either we got a bogus request or
 	 * we need to read and perhaps cache.
 	 */
-	data = read_sha1_file(sha1, &type, &size);
+	data = read_sha1_file(oid->hash, &type, &size);
 	if (!data)
 		return NULL;
 	if (type != OBJ_TREE) {
@@ -1202,7 +1202,7 @@ static struct pbase_tree_cache *pbase_tree_get(const unsigned char *sha1)
 		free(ent->tree_data);
 		nent = ent;
 	}
-	hashcpy(nent->sha1, sha1);
+	oidcpy(&nent->oid, oid);
 	nent->tree_data = data;
 	nent->tree_size = size;
 	nent->ref = 1;
@@ -1247,7 +1247,7 @@ static void add_pbase_object(struct tree_desc *tree,
 		if (cmp < 0)
 			return;
 		if (name[cmplen] != '/') {
-			add_object_entry(entry.oid->hash,
+			add_object_entry(entry.oid,
 					 object_type(entry.mode),
 					 fullname, 1);
 			return;
@@ -1258,7 +1258,7 @@ static void add_pbase_object(struct tree_desc *tree,
 			const char *down = name+cmplen+1;
 			int downlen = name_cmp_len(down);
 
-			tree = pbase_tree_get(entry.oid->hash);
+			tree = pbase_tree_get(entry.oid);
 			if (!tree)
 				return;
 			init_tree_desc(&sub, tree->tree_data, tree->tree_size);
@@ -1317,7 +1317,7 @@ static void add_preferred_base_object(const char *name)
 	cmplen = name_cmp_len(name);
 	for (it = pbase_tree; it; it = it->next) {
 		if (cmplen == 0) {
-			add_object_entry(it->pcache.sha1, OBJ_TREE, NULL, 1);
+			add_object_entry(&it->pcache.oid, OBJ_TREE, NULL, 1);
 		}
 		else {
 			struct tree_desc tree;
@@ -1327,22 +1327,22 @@ static void add_preferred_base_object(const char *name)
 	}
 }
 
-static void add_preferred_base(unsigned char *sha1)
+static void add_preferred_base(struct object_id *oid)
 {
 	struct pbase_tree *it;
 	void *data;
 	unsigned long size;
-	unsigned char tree_sha1[20];
+	struct object_id tree_oid;
 
 	if (window <= num_preferred_base++)
 		return;
 
-	data = read_object_with_reference(sha1, tree_type, &size, tree_sha1);
+	data = read_object_with_reference(oid->hash, tree_type, &size, tree_oid.hash);
 	if (!data)
 		return;
 
 	for (it = pbase_tree; it; it = it->next) {
-		if (!hashcmp(it->pcache.sha1, tree_sha1)) {
+		if (!oidcmp(&it->pcache.oid, &tree_oid)) {
 			free(data);
 			return;
 		}
@@ -1352,7 +1352,7 @@ static void add_preferred_base(unsigned char *sha1)
 	it->next = pbase_tree;
 	pbase_tree = it;
 
-	hashcpy(it->pcache.sha1, tree_sha1);
+	oidcpy(&it->pcache.oid, &tree_oid);
 	it->pcache.tree_data = data;
 	it->pcache.tree_size = size;
 }
@@ -2357,7 +2357,7 @@ static void add_tag_chain(const struct object_id *oid)
 			die("unable to pack objects reachable from tag %s",
 			    oid_to_hex(oid));
 
-		add_object_entry(tag->object.oid.hash, OBJ_TAG, NULL, 0);
+		add_object_entry(&tag->object.oid, OBJ_TAG, NULL, 0);
 
 		if (tag->tagged->type != OBJ_TAG)
 			return;
@@ -2371,7 +2371,7 @@ static int add_ref_tag(const char *path, const struct object_id *oid, int flag,
 	struct object_id peeled;
 
 	if (starts_with(path, "refs/tags/") && /* is a tag? */
-	    !peel_ref(path, peeled.hash)    && /* peelable? */
+	    !peel_ref(path, &peeled)    && /* peelable? */
 	    packlist_find(&to_pack, peeled.hash, NULL))      /* object packed? */
 		add_tag_chain(oid);
 	return 0;
@@ -2505,8 +2505,9 @@ static int git_pack_config(const char *k, const char *v, void *cb)
 
 static void read_object_list_from_stdin(void)
 {
-	char line[40 + 1 + PATH_MAX + 2];
-	unsigned char sha1[20];
+	char line[GIT_MAX_HEXSZ + 1 + PATH_MAX + 2];
+	struct object_id oid;
+	const char *p;
 
 	for (;;) {
 		if (!fgets(line, sizeof(line), stdin)) {
@@ -2520,17 +2521,17 @@ static void read_object_list_from_stdin(void)
 			continue;
 		}
 		if (line[0] == '-') {
-			if (get_sha1_hex(line+1, sha1))
-				die("expected edge sha1, got garbage:\n %s",
+			if (get_oid_hex(line+1, &oid))
+				die("expected edge object ID, got garbage:\n %s",
 				    line);
-			add_preferred_base(sha1);
+			add_preferred_base(&oid);
 			continue;
 		}
-		if (get_sha1_hex(line, sha1))
-			die("expected sha1, got garbage:\n %s", line);
+		if (parse_oid_hex(line, &oid, &p))
+			die("expected object ID, got garbage:\n %s", line);
 
-		add_preferred_base_object(line+41);
-		add_object_entry(sha1, 0, line+41, 0);
+		add_preferred_base_object(p + 1);
+		add_object_entry(&oid, 0, p + 1, 0);
 	}
 }
 
@@ -2538,7 +2539,7 @@ static void read_object_list_from_stdin(void)
 
 static void show_commit(struct commit *commit, void *data)
 {
-	add_object_entry(commit->object.oid.hash, OBJ_COMMIT, NULL, 0);
+	add_object_entry(&commit->object.oid, OBJ_COMMIT, NULL, 0);
 	commit->object.flags |= OBJECT_ADDED;
 
 	if (write_bitmap_index)
@@ -2548,13 +2549,13 @@ static void show_commit(struct commit *commit, void *data)
 static void show_object(struct object *obj, const char *name, void *data)
 {
 	add_preferred_base_object(name);
-	add_object_entry(obj->oid.hash, obj->type, name, 0);
+	add_object_entry(&obj->oid, obj->type, name, 0);
 	obj->flags |= OBJECT_ADDED;
 }
 
 static void show_edge(struct commit *commit)
 {
-	add_preferred_base(commit->object.oid.hash);
+	add_preferred_base(&commit->object.oid);
 }
 
 struct in_pack_object {
@@ -2601,7 +2602,7 @@ static void add_objects_in_unpacked_packs(struct rev_info *revs)
 	memset(&in_pack, 0, sizeof(in_pack));
 
 	for (p = packed_git; p; p = p->next) {
-		const unsigned char *sha1;
+		struct object_id oid;
 		struct object *o;
 
 		if (!p->pack_local || p->pack_keep)
@@ -2614,8 +2615,8 @@ static void add_objects_in_unpacked_packs(struct rev_info *revs)
 			   in_pack.alloc);
 
 		for (i = 0; i < p->num_objects; i++) {
-			sha1 = nth_packed_object_sha1(p, i);
-			o = lookup_unknown_object(sha1);
+			nth_packed_object_oid(&oid, p, i);
+			o = lookup_unknown_object(oid.hash);
 			if (!(o->flags & OBJECT_ADDED))
 				mark_in_pack_object(o, p, &in_pack);
 			o->flags |= OBJECT_ADDED;
@@ -2626,7 +2627,7 @@ static void add_objects_in_unpacked_packs(struct rev_info *revs)
 		QSORT(in_pack.array, in_pack.nr, ofscmp);
 		for (i = 0; i < in_pack.nr; i++) {
 			struct object *o = in_pack.array[i].object;
-			add_object_entry(o->oid.hash, o->type, "", 0);
+			add_object_entry(&o->oid, o->type, "", 0);
 		}
 	}
 	free(in_pack.array);
@@ -2642,7 +2643,7 @@ static int add_loose_object(const struct object_id *oid, const char *path,
 		return 0;
 	}
 
-	add_object_entry(oid->hash, type, "", 0);
+	add_object_entry(oid, type, "", 0);
 	return 0;
 }
 
@@ -2658,7 +2659,7 @@ static void add_unreachable_loose_objects(void)
 				      NULL, NULL, NULL);
 }
 
-static int has_sha1_pack_kept_or_nonlocal(const unsigned char *sha1)
+static int has_sha1_pack_kept_or_nonlocal(const struct object_id *oid)
 {
 	static struct packed_git *last_found = (void *)1;
 	struct packed_git *p;
@@ -2667,7 +2668,7 @@ static int has_sha1_pack_kept_or_nonlocal(const unsigned char *sha1)
 
 	while (p) {
 		if ((!p->pack_local || p->pack_keep) &&
-			find_pack_entry_one(sha1, p)) {
+			find_pack_entry_one(oid->hash, p)) {
 			last_found = p;
 			return 1;
 		}
@@ -2718,7 +2719,7 @@ static void loosen_unused_packed_objects(struct rev_info *revs)
 		for (i = 0; i < p->num_objects; i++) {
 			nth_packed_object_oid(&oid, p, i);
 			if (!packlist_find(&to_pack, oid.hash, NULL) &&
-			    !has_sha1_pack_kept_or_nonlocal(oid.hash) &&
+			    !has_sha1_pack_kept_or_nonlocal(&oid) &&
 			    !loosened_object_can_be_discarded(&oid, p->mtime))
 				if (force_object_loose(oid.hash, p->mtime))
 					die("unable to force loose object");
diff --git a/builtin/pull.c b/builtin/pull.c
index 0413c78a3a..a28f0ffadd 100644
--- a/builtin/pull.c
+++ b/builtin/pull.c
@@ -548,7 +548,7 @@ static int pull_into_void(const struct object_id *merge_head,
 	if (checkout_fast_forward(&empty_tree_oid, merge_head, 0))
 		return 1;
 
-	if (update_ref("initial pull", "HEAD", merge_head->hash, curr_head->hash, 0, UPDATE_REFS_DIE_ON_ERR))
+	if (update_ref("initial pull", "HEAD", merge_head, curr_head, 0, UPDATE_REFS_DIE_ON_ERR))
 		return 1;
 
 	return 0;
diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index cc48767405..4d37a160d7 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -870,7 +870,7 @@ static void refuse_unconfigured_deny_delete_current(void)
 	rp_error("%s", _(refuse_unconfigured_deny_delete_current_msg));
 }
 
-static int command_singleton_iterator(void *cb_data, unsigned char sha1[20]);
+static int command_singleton_iterator(void *cb_data, struct object_id *oid);
 static int update_shallow_ref(struct command *cmd, struct shallow_info *si)
 {
 	static struct lock_file shallow_lock;
@@ -1139,7 +1139,7 @@ static const char *update(struct command *cmd, struct shallow_info *si)
 		}
 		if (ref_transaction_delete(transaction,
 					   namespaced_name,
-					   old_oid ? old_oid->hash : NULL,
+					   old_oid,
 					   0, "push", &err)) {
 			rp_error("%s", err.buf);
 			strbuf_release(&err);
@@ -1156,7 +1156,7 @@ static const char *update(struct command *cmd, struct shallow_info *si)
 
 		if (ref_transaction_update(transaction,
 					   namespaced_name,
-					   new_oid->hash, old_oid->hash,
+					   new_oid, old_oid,
 					   0, "push",
 					   &err)) {
 			rp_error("%s", err.buf);
@@ -1270,7 +1270,7 @@ static void check_aliased_updates(struct command *commands)
 	string_list_clear(&ref_list, 0);
 }
 
-static int command_singleton_iterator(void *cb_data, unsigned char sha1[20])
+static int command_singleton_iterator(void *cb_data, struct object_id *oid)
 {
 	struct command **cmd_list = cb_data;
 	struct command *cmd = *cmd_list;
@@ -1278,7 +1278,7 @@ static int command_singleton_iterator(void *cb_data, unsigned char sha1[20])
 	if (!cmd || is_null_oid(&cmd->new_oid))
 		return -1; /* end of list */
 	*cmd_list = NULL; /* this returns only one */
-	hashcpy(sha1, cmd->new_oid.hash);
+	oidcpy(oid, &cmd->new_oid);
 	return 0;
 }
 
@@ -1309,7 +1309,7 @@ struct iterate_data {
 	struct shallow_info *si;
 };
 
-static int iterate_receive_command_list(void *cb_data, unsigned char sha1[20])
+static int iterate_receive_command_list(void *cb_data, struct object_id *oid)
 {
 	struct iterate_data *data = cb_data;
 	struct command **cmd_list = &data->cmds;
@@ -1320,7 +1320,7 @@ static int iterate_receive_command_list(void *cb_data, unsigned char sha1[20])
 			/* to be checked in update_shallow_ref() */
 			continue;
 		if (!is_null_oid(&cmd->new_oid) && !cmd->skip_update) {
-			hashcpy(sha1, cmd->new_oid.hash);
+			oidcpy(oid, &cmd->new_oid);
 			*cmd_list = cmd->next;
 			return 0;
 		}
diff --git a/builtin/reflog.c b/builtin/reflog.c
index 2067cca5b1..ab31a3b6aa 100644
--- a/builtin/reflog.c
+++ b/builtin/reflog.c
@@ -42,7 +42,7 @@ struct expire_reflog_policy_cb {
 };
 
 struct collected_reflog {
-	unsigned char sha1[20];
+	struct object_id oid;
 	char reflog[FLEX_ARRAY];
 };
 
@@ -385,7 +385,7 @@ static int collect_reflog(const char *ref, const struct object_id *oid, int unus
 	struct collect_reflog_cb *cb = cb_data;
 
 	FLEX_ALLOC_STR(e, reflog, ref);
-	hashcpy(e->sha1, oid->hash);
+	oidcpy(&e->oid, oid);
 	ALLOC_GROW(cb->e, cb->nr + 1, cb->alloc);
 	cb->e[cb->nr++] = e;
 	return 0;
@@ -589,7 +589,7 @@ static int cmd_reflog_expire(int argc, const char **argv, const char *prefix)
 		for (i = 0; i < collected.nr; i++) {
 			struct collected_reflog *e = collected.e[i];
 			set_reflog_expiry_param(&cb.cmd, explicit_expiry, e->reflog);
-			status |= reflog_expire(e->reflog, e->sha1, flags,
+			status |= reflog_expire(e->reflog, &e->oid, flags,
 						reflog_expiry_prepare,
 						should_expire_reflog_ent,
 						reflog_expiry_cleanup,
@@ -601,13 +601,13 @@ static int cmd_reflog_expire(int argc, const char **argv, const char *prefix)
 
 	for (; i < argc; i++) {
 		char *ref;
-		unsigned char sha1[20];
-		if (!dwim_log(argv[i], strlen(argv[i]), sha1, &ref)) {
+		struct object_id oid;
+		if (!dwim_log(argv[i], strlen(argv[i]), &oid, &ref)) {
 			status |= error("%s points nowhere!", argv[i]);
 			continue;
 		}
 		set_reflog_expiry_param(&cb.cmd, explicit_expiry, ref);
-		status |= reflog_expire(ref, sha1, flags,
+		status |= reflog_expire(ref, &oid, flags,
 					reflog_expiry_prepare,
 					should_expire_reflog_ent,
 					reflog_expiry_cleanup,
@@ -659,7 +659,7 @@ static int cmd_reflog_delete(int argc, const char **argv, const char *prefix)
 
 	for ( ; i < argc; i++) {
 		const char *spec = strstr(argv[i], "@{");
-		unsigned char sha1[20];
+		struct object_id oid;
 		char *ep, *ref;
 		int recno;
 
@@ -668,7 +668,7 @@ static int cmd_reflog_delete(int argc, const char **argv, const char *prefix)
 			continue;
 		}
 
-		if (!dwim_log(argv[i], spec - argv[i], sha1, &ref)) {
+		if (!dwim_log(argv[i], spec - argv[i], &oid, &ref)) {
 			status |= error("no reflog for '%s'", argv[i]);
 			continue;
 		}
@@ -683,7 +683,7 @@ static int cmd_reflog_delete(int argc, const char **argv, const char *prefix)
 			cb.cmd.expire_total = 0;
 		}
 
-		status |= reflog_expire(ref, sha1, flags,
+		status |= reflog_expire(ref, &oid, flags,
 					reflog_expiry_prepare,
 					should_expire_reflog_ent,
 					reflog_expiry_cleanup,
diff --git a/builtin/remote.c b/builtin/remote.c
index bc89623695..a04ea50e40 100644
--- a/builtin/remote.c
+++ b/builtin/remote.c
@@ -690,7 +690,7 @@ static int mv(int argc, const char **argv)
 		int flag = 0;
 		struct object_id oid;
 
-		read_ref_full(item->string, RESOLVE_REF_READING, oid.hash, &flag);
+		read_ref_full(item->string, RESOLVE_REF_READING, &oid, &flag);
 		if (!(flag & REF_ISSYMREF))
 			continue;
 		if (delete_ref(NULL, item->string, NULL, REF_NODEREF))
diff --git a/builtin/replace.c b/builtin/replace.c
index 3e71a77152..10078ae371 100644
--- a/builtin/replace.c
+++ b/builtin/replace.c
@@ -113,7 +113,7 @@ static int for_each_replace_name(const char **argv, each_replace_name_fn fn)
 		strbuf_addstr(&ref, oid_to_hex(&oid));
 		full_hex = ref.buf + base_len;
 
-		if (read_ref(ref.buf, oid.hash)) {
+		if (read_ref(ref.buf, &oid)) {
 			error("replace ref '%s' not found.", full_hex);
 			had_error = 1;
 			continue;
@@ -128,7 +128,7 @@ static int for_each_replace_name(const char **argv, each_replace_name_fn fn)
 static int delete_replace_ref(const char *name, const char *ref,
 			      const struct object_id *oid)
 {
-	if (delete_ref(NULL, ref, oid->hash, 0))
+	if (delete_ref(NULL, ref, oid, 0))
 		return 1;
 	printf("Deleted replace ref '%s'\n", name);
 	return 0;
@@ -144,7 +144,7 @@ static void check_ref_valid(struct object_id *object,
 	if (check_refname_format(ref->buf, 0))
 		die("'%s' is not a valid ref name.", ref->buf);
 
-	if (read_ref(ref->buf, prev->hash))
+	if (read_ref(ref->buf, prev))
 		oidclr(prev);
 	else if (!force)
 		die("replace ref '%s' already exists", ref->buf);
@@ -175,7 +175,7 @@ static int replace_object_oid(const char *object_ref,
 
 	transaction = ref_transaction_begin(&err);
 	if (!transaction ||
-	    ref_transaction_update(transaction, ref.buf, repl->hash, prev.hash,
+	    ref_transaction_update(transaction, ref.buf, repl, &prev,
 				   0, NULL, &err) ||
 	    ref_transaction_commit(transaction, &err))
 		die("%s", err.buf);
diff --git a/builtin/reset.c b/builtin/reset.c
index 9cd89b2305..d4003f76ab 100644
--- a/builtin/reset.c
+++ b/builtin/reset.c
@@ -266,12 +266,12 @@ static int reset_refs(const char *rev, const struct object_id *oid)
 	if (!get_oid("HEAD", &oid_orig)) {
 		orig = &oid_orig;
 		set_reflog_message(&msg, "updating ORIG_HEAD", NULL);
-		update_ref_oid(msg.buf, "ORIG_HEAD", orig, old_orig, 0,
+		update_ref(msg.buf, "ORIG_HEAD", orig, old_orig, 0,
 			   UPDATE_REFS_MSG_ON_ERR);
 	} else if (old_orig)
-		delete_ref(NULL, "ORIG_HEAD", old_orig->hash, 0);
+		delete_ref(NULL, "ORIG_HEAD", old_orig, 0);
 	set_reflog_message(&msg, "updating HEAD", rev);
-	update_ref_status = update_ref_oid(msg.buf, "HEAD", oid, orig, 0,
+	update_ref_status = update_ref(msg.buf, "HEAD", oid, orig, 0,
 				       UPDATE_REFS_MSG_ON_ERR);
 	strbuf_release(&msg);
 	return update_ref_status;
diff --git a/builtin/rev-list.c b/builtin/rev-list.c
index c1c74d4a79..9bf8d5991c 100644
--- a/builtin/rev-list.c
+++ b/builtin/rev-list.c
@@ -258,14 +258,14 @@ static int show_bisect_vars(struct rev_list_info *info, int reaches, int all)
 }
 
 static int show_object_fast(
-	const unsigned char *sha1,
+	const struct object_id *oid,
 	enum object_type type,
 	int exclude,
 	uint32_t name_hash,
 	struct packed_git *found_pack,
 	off_t found_offset)
 {
-	fprintf(stdout, "%s\n", sha1_to_hex(sha1));
+	fprintf(stdout, "%s\n", oid_to_hex(oid));
 	return 1;
 }
 
diff --git a/builtin/rev-parse.c b/builtin/rev-parse.c
index a8d7e6f7ae..74aa644cbb 100644
--- a/builtin/rev-parse.c
+++ b/builtin/rev-parse.c
@@ -133,7 +133,7 @@ static void show_rev(int type, const struct object_id *oid, const char *name)
 			struct object_id discard;
 			char *full;
 
-			switch (dwim_ref(name, strlen(name), discard.hash, &full)) {
+			switch (dwim_ref(name, strlen(name), &discard, &full)) {
 			case 0:
 				/*
 				 * Not found -- not a ref.  We could
diff --git a/builtin/show-branch.c b/builtin/show-branch.c
index 6fa1f62a88..2e24b5c330 100644
--- a/builtin/show-branch.c
+++ b/builtin/show-branch.c
@@ -705,8 +705,8 @@ int cmd_show_branch(int ac, const char **av, const char *prefix)
 			static const char *fake_av[2];
 
 			fake_av[0] = resolve_refdup("HEAD",
-						    RESOLVE_REF_READING,
-						    oid.hash, NULL);
+						    RESOLVE_REF_READING, &oid,
+						    NULL);
 			fake_av[1] = NULL;
 			av = fake_av;
 			ac = 1;
@@ -720,7 +720,7 @@ int cmd_show_branch(int ac, const char **av, const char *prefix)
 			die(Q_("only %d entry can be shown at one time.",
 			       "only %d entries can be shown at one time.",
 			       MAX_REVS), MAX_REVS);
-		if (!dwim_ref(*av, strlen(*av), oid.hash, &ref))
+		if (!dwim_ref(*av, strlen(*av), &oid, &ref))
 			die(_("no such ref %s"), *av);
 
 		/* Has the base been specified? */
@@ -731,7 +731,7 @@ int cmd_show_branch(int ac, const char **av, const char *prefix)
 				/* Ah, that is a date spec... */
 				timestamp_t at;
 				at = approxidate(reflog_base);
-				read_ref_at(ref, flags, at, -1, oid.hash, NULL,
+				read_ref_at(ref, flags, at, -1, &oid, NULL,
 					    NULL, NULL, &base);
 			}
 		}
@@ -743,7 +743,7 @@ int cmd_show_branch(int ac, const char **av, const char *prefix)
 			timestamp_t timestamp;
 			int tz;
 
-			if (read_ref_at(ref, flags, 0, base+i, oid.hash, &logmsg,
+			if (read_ref_at(ref, flags, 0, base + i, &oid, &logmsg,
 					&timestamp, &tz, NULL)) {
 				reflog = i;
 				break;
@@ -775,7 +775,7 @@ int cmd_show_branch(int ac, const char **av, const char *prefix)
 	}
 
 	head = resolve_refdup("HEAD", RESOLVE_REF_READING,
-			      head_oid.hash, NULL);
+			      &head_oid, NULL);
 
 	if (with_current_branch && head) {
 		int has_head = 0;
diff --git a/builtin/show-ref.c b/builtin/show-ref.c
index 013d241abc..41e5e71cad 100644
--- a/builtin/show-ref.c
+++ b/builtin/show-ref.c
@@ -38,7 +38,7 @@ static void show_one(const char *refname, const struct object_id *oid)
 	if (!deref_tags)
 		return;
 
-	if (!peel_ref(refname, peeled.hash)) {
+	if (!peel_ref(refname, &peeled)) {
 		hex = find_unique_abbrev(peeled.hash, abbrev);
 		printf("%s %s^{}\n", hex, refname);
 	}
@@ -197,7 +197,7 @@ int cmd_show_ref(int argc, const char **argv, const char *prefix)
 			struct object_id oid;
 
 			if ((starts_with(*pattern, "refs/") || !strcmp(*pattern, "HEAD")) &&
-			    !read_ref(*pattern, oid.hash)) {
+			    !read_ref(*pattern, &oid)) {
 				show_one(*pattern, &oid);
 			}
 			else if (!quiet)
diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
index d366e8e7b3..2086f0eb08 100644
--- a/builtin/submodule--helper.c
+++ b/builtin/submodule--helper.c
@@ -1382,7 +1382,7 @@ static int push_check(int argc, const char **argv, const char *prefix)
 	argv++;
 	argc--;
 	/* Get the submodule's head ref and determine if it is detached */
-	head = resolve_refdup("HEAD", 0, head_oid.hash, NULL);
+	head = resolve_refdup("HEAD", 0, &head_oid, NULL);
 	if (!head)
 		die(_("Failed to resolve HEAD as a valid ref."));
 	if (!strcmp(head, "HEAD"))
diff --git a/builtin/tag.c b/builtin/tag.c
index b38329b593..a7e6a5b0f2 100644
--- a/builtin/tag.c
+++ b/builtin/tag.c
@@ -82,7 +82,7 @@ static int for_each_tag_name(const char **argv, each_tag_name_fn fn,
 	for (p = argv; *p; p++) {
 		strbuf_reset(&ref);
 		strbuf_addf(&ref, "refs/tags/%s", *p);
-		if (read_ref(ref.buf, oid.hash)) {
+		if (read_ref(ref.buf, &oid)) {
 			error(_("tag '%s' not found."), *p);
 			had_error = 1;
 			continue;
@@ -97,7 +97,7 @@ static int for_each_tag_name(const char **argv, each_tag_name_fn fn,
 static int delete_tag(const char *name, const char *ref,
 		      const struct object_id *oid, const void *cb_data)
 {
-	if (delete_ref(NULL, ref, oid->hash, 0))
+	if (delete_ref(NULL, ref, oid, 0))
 		return 1;
 	printf(_("Deleted tag '%s' (was %s)\n"), name, find_unique_abbrev(oid->hash, DEFAULT_ABBREV));
 	return 0;
@@ -518,7 +518,7 @@ int cmd_tag(int argc, const char **argv, const char *prefix)
 	if (strbuf_check_tag_ref(&ref, tag))
 		die(_("'%s' is not a valid tag name."), tag);
 
-	if (read_ref(ref.buf, prev.hash))
+	if (read_ref(ref.buf, &prev))
 		oidclr(&prev);
 	else if (!force)
 		die(_("tag '%s' already exists"), tag);
@@ -544,7 +544,7 @@ int cmd_tag(int argc, const char **argv, const char *prefix)
 
 	transaction = ref_transaction_begin(&err);
 	if (!transaction ||
-	    ref_transaction_update(transaction, ref.buf, object.hash, prev.hash,
+	    ref_transaction_update(transaction, ref.buf, &object, &prev,
 				   create_reflog ? REF_FORCE_CREATE_REFLOG : 0,
 				   reflog_msg.buf, &err) ||
 	    ref_transaction_commit(transaction, &err))
diff --git a/builtin/update-index.c b/builtin/update-index.c
index bf7420b808..fefbe60167 100644
--- a/builtin/update-index.c
+++ b/builtin/update-index.c
@@ -328,7 +328,7 @@ static int process_directory(const char *path, int len, struct stat *st)
 		if (S_ISGITLINK(ce->ce_mode)) {
 
 			/* Do nothing to the index if there is no HEAD! */
-			if (resolve_gitlink_ref(path, "HEAD", oid.hash) < 0)
+			if (resolve_gitlink_ref(path, "HEAD", &oid) < 0)
 				return 0;
 
 			return add_one_path(ce, path, len, st);
@@ -354,7 +354,7 @@ static int process_directory(const char *path, int len, struct stat *st)
 	}
 
 	/* No match - should we add it as a gitlink? */
-	if (!resolve_gitlink_ref(path, "HEAD", oid.hash))
+	if (!resolve_gitlink_ref(path, "HEAD", &oid))
 		return add_one_path(NULL, path, len, st);
 
 	/* Error out. */
@@ -679,9 +679,9 @@ static int unresolve_one(const char *path)
 
 static void read_head_pointers(void)
 {
-	if (read_ref("HEAD", head_oid.hash))
+	if (read_ref("HEAD", &head_oid))
 		die("No HEAD -- no initial commit yet?");
-	if (read_ref("MERGE_HEAD", merge_head_oid.hash)) {
+	if (read_ref("MERGE_HEAD", &merge_head_oid)) {
 		fprintf(stderr, "Not in the middle of a merge.\n");
 		exit(0);
 	}
@@ -721,7 +721,7 @@ static int do_reupdate(int ac, const char **av,
 		       PATHSPEC_PREFER_CWD,
 		       prefix, av + 1);
 
-	if (read_ref("HEAD", head_oid.hash))
+	if (read_ref("HEAD", &head_oid))
 		/* If there is no HEAD, that means it is an initial
 		 * commit.  Update everything in the index.
 		 */
diff --git a/builtin/update-ref.c b/builtin/update-ref.c
index 6b90c5dead..cf1552b478 100644
--- a/builtin/update-ref.c
+++ b/builtin/update-ref.c
@@ -200,7 +200,7 @@ static const char *parse_cmd_update(struct ref_transaction *transaction,
 		die("update %s: extra input: %s", refname, next);
 
 	if (ref_transaction_update(transaction, refname,
-				   new_oid.hash, have_old ? old_oid.hash : NULL,
+				   &new_oid, have_old ? &old_oid : NULL,
 				   update_flags | create_reflog_flag,
 				   msg, &err))
 		die("%s", err.buf);
@@ -232,7 +232,7 @@ static const char *parse_cmd_create(struct ref_transaction *transaction,
 	if (*next != line_termination)
 		die("create %s: extra input: %s", refname, next);
 
-	if (ref_transaction_create(transaction, refname, new_oid.hash,
+	if (ref_transaction_create(transaction, refname, &new_oid,
 				   update_flags | create_reflog_flag,
 				   msg, &err))
 		die("%s", err.buf);
@@ -269,7 +269,7 @@ static const char *parse_cmd_delete(struct ref_transaction *transaction,
 		die("delete %s: extra input: %s", refname, next);
 
 	if (ref_transaction_delete(transaction, refname,
-				   have_old ? old_oid.hash : NULL,
+				   have_old ? &old_oid : NULL,
 				   update_flags, msg, &err))
 		die("%s", err.buf);
 
@@ -298,7 +298,7 @@ static const char *parse_cmd_verify(struct ref_transaction *transaction,
 	if (*next != line_termination)
 		die("verify %s: extra input: %s", refname, next);
 
-	if (ref_transaction_verify(transaction, refname, old_oid.hash,
+	if (ref_transaction_verify(transaction, refname, &old_oid,
 				   update_flags, &err))
 		die("%s", err.buf);
 
@@ -434,10 +434,10 @@ int cmd_update_ref(int argc, const char **argv, const char *prefix)
 		 * NULL_SHA1 as "don't care" here:
 		 */
 		return delete_ref(msg, refname,
-				  (oldval && !is_null_oid(&oldoid)) ? oldoid.hash : NULL,
+				  (oldval && !is_null_oid(&oldoid)) ? &oldoid : NULL,
 				  flags);
 	else
-		return update_ref(msg, refname, oid.hash, oldval ? oldoid.hash : NULL,
+		return update_ref(msg, refname, &oid, oldval ? &oldoid : NULL,
 				  flags | create_reflog_flag,
 				  UPDATE_REFS_DIE_ON_ERR);
 }
diff --git a/builtin/worktree.c b/builtin/worktree.c
index 7b9307aa58..ed043d5f1c 100644
--- a/builtin/worktree.c
+++ b/builtin/worktree.c
@@ -410,7 +410,7 @@ static void show_worktree_porcelain(struct worktree *wt)
 	if (wt->is_bare)
 		printf("bare\n");
 	else {
-		printf("HEAD %s\n", sha1_to_hex(wt->head_sha1));
+		printf("HEAD %s\n", oid_to_hex(&wt->head_oid));
 		if (wt->is_detached)
 			printf("detached\n");
 		else if (wt->head_ref)
@@ -430,7 +430,7 @@ static void show_worktree(struct worktree *wt, int path_maxlen, int abbrev_len)
 		strbuf_addstr(&sb, "(bare)");
 	else {
 		strbuf_addf(&sb, "%-*s ", abbrev_len,
-				find_unique_abbrev(wt->head_sha1, DEFAULT_ABBREV));
+				find_unique_abbrev(wt->head_oid.hash, DEFAULT_ABBREV));
 		if (wt->is_detached)
 			strbuf_addstr(&sb, "(detached HEAD)");
 		else if (wt->head_ref) {
@@ -455,7 +455,7 @@ static void measure_widths(struct worktree **wt, int *abbrev, int *maxlen)
 
 		if (path_len > *maxlen)
 			*maxlen = path_len;
-		sha1_len = strlen(find_unique_abbrev(wt[i]->head_sha1, *abbrev));
+		sha1_len = strlen(find_unique_abbrev(wt[i]->head_oid.hash, *abbrev));
 		if (sha1_len > *abbrev)
 			*abbrev = sha1_len;
 	}
diff --git a/bundle.c b/bundle.c
index c092d5d68f..93290962c9 100644
--- a/bundle.c
+++ b/bundle.c
@@ -338,9 +338,9 @@ static int write_bundle_refs(int bundle_fd, struct rev_info *revs)
 
 		if (e->item->flags & UNINTERESTING)
 			continue;
-		if (dwim_ref(e->name, strlen(e->name), oid.hash, &ref) != 1)
+		if (dwim_ref(e->name, strlen(e->name), &oid, &ref) != 1)
 			goto skip_write_ref;
-		if (read_ref_full(e->name, RESOLVE_REF_READING, oid.hash, &flag))
+		if (read_ref_full(e->name, RESOLVE_REF_READING, &oid, &flag))
 			flag = 0;
 		display_ref = (flag & REF_ISSYMREF) ? e->name : ref;
 
diff --git a/combine-diff.c b/combine-diff.c
index 9e163d5ada..82f6070977 100644
--- a/combine-diff.c
+++ b/combine-diff.c
@@ -1014,7 +1014,7 @@ static void show_patch_diff(struct combine_diff_path *elem, int num_parent,
 			elem->mode = canon_mode(st.st_mode);
 		} else if (S_ISDIR(st.st_mode)) {
 			struct object_id oid;
-			if (resolve_gitlink_ref(elem->path, "HEAD", oid.hash) < 0)
+			if (resolve_gitlink_ref(elem->path, "HEAD", &oid) < 0)
 				result = grab_blob(&elem->oid, elem->mode,
 						   &result_size, NULL, NULL);
 			else
diff --git a/connected.c b/connected.c
index f416b05051..4a47f33270 100644
--- a/connected.c
+++ b/connected.c
@@ -16,13 +16,13 @@
  *
  * Returns 0 if everything is connected, non-zero otherwise.
  */
-int check_connected(sha1_iterate_fn fn, void *cb_data,
+int check_connected(oid_iterate_fn fn, void *cb_data,
 		    struct check_connected_options *opt)
 {
 	struct child_process rev_list = CHILD_PROCESS_INIT;
 	struct check_connected_options defaults = CHECK_CONNECTED_INIT;
-	char commit[41];
-	unsigned char sha1[20];
+	char commit[GIT_MAX_HEXSZ + 1];
+	struct object_id oid;
 	int err = 0;
 	struct packed_git *new_pack = NULL;
 	struct transport *transport;
@@ -32,7 +32,7 @@ int check_connected(sha1_iterate_fn fn, void *cb_data,
 		opt = &defaults;
 	transport = opt->transport;
 
-	if (fn(cb_data, sha1)) {
+	if (fn(cb_data, &oid)) {
 		if (opt->err_fd)
 			close(opt->err_fd);
 		return err;
@@ -77,7 +77,7 @@ int check_connected(sha1_iterate_fn fn, void *cb_data,
 
 	sigchain_push(SIGPIPE, SIG_IGN);
 
-	commit[40] = '\n';
+	commit[GIT_SHA1_HEXSZ] = '\n';
 	do {
 		/*
 		 * If index-pack already checked that:
@@ -87,17 +87,17 @@ int check_connected(sha1_iterate_fn fn, void *cb_data,
 		 * are sure the ref is good and not sending it to
 		 * rev-list for verification.
 		 */
-		if (new_pack && find_pack_entry_one(sha1, new_pack))
+		if (new_pack && find_pack_entry_one(oid.hash, new_pack))
 			continue;
 
-		memcpy(commit, sha1_to_hex(sha1), 40);
-		if (write_in_full(rev_list.in, commit, 41) < 0) {
+		memcpy(commit, oid_to_hex(&oid), GIT_SHA1_HEXSZ);
+		if (write_in_full(rev_list.in, commit, GIT_SHA1_HEXSZ + 1) < 0) {
 			if (errno != EPIPE && errno != EINVAL)
 				error_errno(_("failed write to rev-list"));
 			err = -1;
 			break;
 		}
-	} while (!fn(cb_data, sha1));
+	} while (!fn(cb_data, &oid));
 
 	if (close(rev_list.in))
 		err = error_errno(_("failed to close rev-list's stdin"));
diff --git a/connected.h b/connected.h
index 4ca325f79d..a53f03a61a 100644
--- a/connected.h
+++ b/connected.h
@@ -8,7 +8,7 @@ struct transport;
  * When called after returning the name for the last object, return -1
  * to signal EOF, otherwise return 0.
  */
-typedef int (*sha1_iterate_fn)(void *, unsigned char [20]);
+typedef int (*oid_iterate_fn)(void *, struct object_id *oid);
 
 /*
  * Named-arguments struct for check_connected. All arguments are
@@ -51,7 +51,7 @@ struct check_connected_options {
  *
  * If "opt" is NULL, behaves as if CHECK_CONNECTED_INIT was passed.
  */
-int check_connected(sha1_iterate_fn fn, void *cb_data,
+int check_connected(oid_iterate_fn fn, void *cb_data,
 		    struct check_connected_options *opt);
 
 #endif /* CONNECTED_H */
diff --git a/diff-lib.c b/diff-lib.c
index 4e0980caa8..d2ea02f4d7 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -36,7 +36,7 @@ static int check_removed(const struct cache_entry *ce, struct stat *st)
 	if (has_symlink_leading_path(ce->name, ce_namelen(ce)))
 		return 1;
 	if (S_ISDIR(st->st_mode)) {
-		unsigned char sub[20];
+		struct object_id sub;
 
 		/*
 		 * If ce is already a gitlink, we can have a plain
@@ -50,7 +50,7 @@ static int check_removed(const struct cache_entry *ce, struct stat *st)
 		 * a directory --- the blob was removed!
 		 */
 		if (!S_ISGITLINK(ce->ce_mode) &&
-		    resolve_gitlink_ref(ce->name, "HEAD", sub))
+		    resolve_gitlink_ref(ce->name, "HEAD", &sub))
 			return 1;
 	}
 	return 0;
diff --git a/dir.c b/dir.c
index 9987011da5..fc2bdc79fc 100644
--- a/dir.c
+++ b/dir.c
@@ -1390,8 +1390,8 @@ static enum path_treatment treat_directory(struct dir_struct *dir,
 		if (dir->flags & DIR_SHOW_OTHER_DIRECTORIES)
 			break;
 		if (!(dir->flags & DIR_NO_GITLINKS)) {
-			unsigned char sha1[20];
-			if (resolve_gitlink_ref(dirname, "HEAD", sha1) == 0)
+			struct object_id oid;
+			if (resolve_gitlink_ref(dirname, "HEAD", &oid) == 0)
 				return exclude ? path_excluded : path_untracked;
 		}
 		return path_recurse;
@@ -2279,10 +2279,10 @@ static int remove_dir_recurse(struct strbuf *path, int flag, int *kept_up)
 	int ret = 0, original_len = path->len, len, kept_down = 0;
 	int only_empty = (flag & REMOVE_DIR_EMPTY_ONLY);
 	int keep_toplevel = (flag & REMOVE_DIR_KEEP_TOPLEVEL);
-	unsigned char submodule_head[20];
+	struct object_id submodule_head;
 
 	if ((flag & REMOVE_DIR_KEEP_NESTED_GIT) &&
-	    !resolve_gitlink_ref(path->buf, "HEAD", submodule_head)) {
+	    !resolve_gitlink_ref(path->buf, "HEAD", &submodule_head)) {
 		/* Do not descend and nuke a nested git work tree. */
 		if (kept_up)
 			*kept_up = 1;
diff --git a/fast-import.c b/fast-import.c
index d5e4cf0bad..b70ac025e0 100644
--- a/fast-import.c
+++ b/fast-import.c
@@ -1758,7 +1758,7 @@ static int update_branch(struct branch *b)
 			delete_ref(NULL, b->name, NULL, 0);
 		return 0;
 	}
-	if (read_ref(b->name, old_oid.hash))
+	if (read_ref(b->name, &old_oid))
 		oidclr(&old_oid);
 	if (!force_update && !is_null_oid(&old_oid)) {
 		struct commit *old_cmit, *new_cmit;
@@ -1778,7 +1778,7 @@ static int update_branch(struct branch *b)
 	}
 	transaction = ref_transaction_begin(&err);
 	if (!transaction ||
-	    ref_transaction_update(transaction, b->name, b->oid.hash, old_oid.hash,
+	    ref_transaction_update(transaction, b->name, &b->oid, &old_oid,
 				   0, msg, &err) ||
 	    ref_transaction_commit(transaction, &err)) {
 		ref_transaction_free(transaction);
@@ -1820,7 +1820,7 @@ static void dump_tags(void)
 		strbuf_addf(&ref_name, "refs/tags/%s", t->name);
 
 		if (ref_transaction_update(transaction, ref_name.buf,
-					   t->oid.hash, NULL, 0, msg, &err)) {
+					   &t->oid, NULL, 0, msg, &err)) {
 			failure |= error("%s", err.buf);
 			goto cleanup;
 		}
diff --git a/notes-cache.c b/notes-cache.c
index 29b4cede5f..17ee8602b3 100644
--- a/notes-cache.c
+++ b/notes-cache.c
@@ -11,7 +11,7 @@ static int notes_cache_match_validity(const char *ref, const char *validity)
 	struct strbuf msg = STRBUF_INIT;
 	int ret;
 
-	if (read_ref(ref, oid.hash) < 0)
+	if (read_ref(ref, &oid) < 0)
 		return 0;
 
 	commit = lookup_commit_reference_gently(&oid, 1);
@@ -59,7 +59,7 @@ int notes_cache_write(struct notes_cache *c)
 	if (commit_tree(c->validity, strlen(c->validity), tree_oid.hash, NULL,
 			commit_oid.hash, NULL, NULL) < 0)
 		return -1;
-	if (update_ref("update notes cache", c->tree.update_ref, commit_oid.hash,
+	if (update_ref("update notes cache", c->tree.update_ref, &commit_oid,
 		       NULL, 0, UPDATE_REFS_QUIET_ON_ERR) < 0)
 		return -1;
 
diff --git a/notes-merge.c b/notes-merge.c
index 4352c34a6e..30ec83ab04 100644
--- a/notes-merge.c
+++ b/notes-merge.c
@@ -547,7 +547,7 @@ int notes_merge(struct notes_merge_options *o,
 	       o->local_ref, o->remote_ref);
 
 	/* Dereference o->local_ref into local_sha1 */
-	if (read_ref_full(o->local_ref, 0, local_oid.hash, NULL))
+	if (read_ref_full(o->local_ref, 0, &local_oid, NULL))
 		die("Failed to resolve local notes ref '%s'", o->local_ref);
 	else if (!check_refname_format(o->local_ref, 0) &&
 		is_null_oid(&local_oid))
diff --git a/notes-utils.c b/notes-utils.c
index 9765deb41a..5c8e70c98f 100644
--- a/notes-utils.c
+++ b/notes-utils.c
@@ -18,7 +18,7 @@ void create_notes_commit(struct notes_tree *t, struct commit_list *parents,
 	if (!parents) {
 		/* Deduce parent commit from t->ref */
 		struct object_id parent_oid;
-		if (!read_ref(t->ref, parent_oid.hash)) {
+		if (!read_ref(t->ref, &parent_oid)) {
 			struct commit *parent = lookup_commit(&parent_oid);
 			if (parse_commit(parent))
 				die("Failed to find/parse commit %s", t->ref);
@@ -49,7 +49,7 @@ void commit_notes(struct notes_tree *t, const char *msg)
 
 	create_notes_commit(t, NULL, buf.buf, buf.len, commit_oid.hash);
 	strbuf_insert(&buf, 0, "notes: ", 7); /* commit message starts at index 7 */
-	update_ref(buf.buf, t->update_ref, commit_oid.hash, NULL, 0,
+	update_ref(buf.buf, t->update_ref, &commit_oid, NULL, 0,
 		   UPDATE_REFS_DIE_ON_ERR);
 
 	strbuf_release(&buf);
diff --git a/notes.c b/notes.c
index 5c62862574..d273822b28 100644
--- a/notes.c
+++ b/notes.c
@@ -1027,7 +1027,7 @@ void init_notes(struct notes_tree *t, const char *notes_ref,
 	if (flags & NOTES_INIT_EMPTY || !notes_ref ||
 	    get_oid_treeish(notes_ref, &object_oid))
 		return;
-	if (flags & NOTES_INIT_WRITABLE && read_ref(notes_ref, object_oid.hash))
+	if (flags & NOTES_INIT_WRITABLE && read_ref(notes_ref, &object_oid))
 		die("Cannot use notes ref %s", notes_ref);
 	if (get_tree_entry(object_oid.hash, "", oid.hash, &mode))
 		die("Failed to read notes tree referenced by %s (%s)",
diff --git a/pack-bitmap.c b/pack-bitmap.c
index 42e3d5f4f2..9270983e5f 100644
--- a/pack-bitmap.c
+++ b/pack-bitmap.c
@@ -587,7 +587,7 @@ static void show_extended_objects(struct bitmap *objects,
 			continue;
 
 		obj = eindex->objects[i];
-		show_reach(obj->oid.hash, obj->type, 0, eindex->hashes[i], NULL, 0);
+		show_reach(&obj->oid, obj->type, 0, eindex->hashes[i], NULL, 0);
 	}
 }
 
@@ -612,7 +612,7 @@ static void show_objects_for_type(
 		eword_t word = objects->words[i] & filter;
 
 		for (offset = 0; offset < BITS_IN_EWORD; ++offset) {
-			const unsigned char *sha1;
+			struct object_id oid;
 			struct revindex_entry *entry;
 			uint32_t hash = 0;
 
@@ -625,12 +625,12 @@ static void show_objects_for_type(
 				continue;
 
 			entry = &bitmap_git.pack->revindex[pos + offset];
-			sha1 = nth_packed_object_sha1(bitmap_git.pack, entry->nr);
+			nth_packed_object_oid(&oid, bitmap_git.pack, entry->nr);
 
 			if (bitmap_git.hashes)
 				hash = get_be32(bitmap_git.hashes + entry->nr);
 
-			show_reach(sha1, object_type, 0, hash, bitmap_git.pack, entry->offset);
+			show_reach(&oid, object_type, 0, hash, bitmap_git.pack, entry->offset);
 		}
 
 		pos += BITS_IN_EWORD;
diff --git a/pack-bitmap.h b/pack-bitmap.h
index 0adcef77b5..3742a00e14 100644
--- a/pack-bitmap.h
+++ b/pack-bitmap.h
@@ -27,7 +27,7 @@ enum pack_bitmap_flags {
 };
 
 typedef int (*show_reachable_fn)(
-	const unsigned char *sha1,
+	const struct object_id *oid,
 	enum object_type type,
 	int flags,
 	uint32_t hash,
diff --git a/read-cache.c b/read-cache.c
index 87188d390d..f3d125c114 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -191,7 +191,7 @@ static int ce_compare_link(const struct cache_entry *ce, size_t expected_size)
 
 static int ce_compare_gitlink(const struct cache_entry *ce)
 {
-	unsigned char sha1[20];
+	struct object_id oid;
 
 	/*
 	 * We don't actually require that the .git directory
@@ -201,9 +201,9 @@ static int ce_compare_gitlink(const struct cache_entry *ce)
 	 *
 	 * If so, we consider it always to match.
 	 */
-	if (resolve_gitlink_ref(ce->name, "HEAD", sha1) < 0)
+	if (resolve_gitlink_ref(ce->name, "HEAD", &oid) < 0)
 		return 0;
-	return hashcmp(sha1, ce->oid.hash);
+	return oidcmp(&oid, &ce->oid);
 }
 
 static int ce_modified_check_fs(const struct cache_entry *ce, struct stat *st)
diff --git a/reflog-walk.c b/reflog-walk.c
index 842b2f77dc..5008bbf6ad 100644
--- a/reflog-walk.c
+++ b/reflog-walk.c
@@ -161,7 +161,7 @@ int add_reflog_for_walk(struct reflog_walk_info *info,
 			struct object_id oid;
 			char *b;
 			int ret = dwim_log(branch, strlen(branch),
-					   oid.hash, &b);
+					   &oid, &b);
 			if (ret > 1)
 				free(b);
 			else if (ret == 1) {
diff --git a/refs.c b/refs.c
index c590a992fb..62a7621025 100644
--- a/refs.c
+++ b/refs.c
@@ -194,21 +194,21 @@ int ref_resolves_to_object(const char *refname,
 
 char *refs_resolve_refdup(struct ref_store *refs,
 			  const char *refname, int resolve_flags,
-			  unsigned char *sha1, int *flags)
+			  struct object_id *oid, int *flags)
 {
 	const char *result;
 
 	result = refs_resolve_ref_unsafe(refs, refname, resolve_flags,
-					 sha1, flags);
+					 oid, flags);
 	return xstrdup_or_null(result);
 }
 
 char *resolve_refdup(const char *refname, int resolve_flags,
-		     unsigned char *sha1, int *flags)
+		     struct object_id *oid, int *flags)
 {
 	return refs_resolve_refdup(get_main_ref_store(),
 				   refname, resolve_flags,
-				   sha1, flags);
+				   oid, flags);
 }
 
 /* The argument to filter_refs */
@@ -219,22 +219,22 @@ struct ref_filter {
 };
 
 int refs_read_ref_full(struct ref_store *refs, const char *refname,
-		       int resolve_flags, unsigned char *sha1, int *flags)
+		       int resolve_flags, struct object_id *oid, int *flags)
 {
-	if (refs_resolve_ref_unsafe(refs, refname, resolve_flags, sha1, flags))
+	if (refs_resolve_ref_unsafe(refs, refname, resolve_flags, oid, flags))
 		return 0;
 	return -1;
 }
 
-int read_ref_full(const char *refname, int resolve_flags, unsigned char *sha1, int *flags)
+int read_ref_full(const char *refname, int resolve_flags, struct object_id *oid, int *flags)
 {
 	return refs_read_ref_full(get_main_ref_store(), refname,
-				  resolve_flags, sha1, flags);
+				  resolve_flags, oid, flags);
 }
 
-int read_ref(const char *refname, unsigned char *sha1)
+int read_ref(const char *refname, struct object_id *oid)
 {
-	return read_ref_full(refname, RESOLVE_REF_READING, sha1, NULL);
+	return read_ref_full(refname, RESOLVE_REF_READING, oid, NULL);
 }
 
 int ref_exists(const char *refname)
@@ -252,12 +252,12 @@ static int filter_refs(const char *refname, const struct object_id *oid,
 	return filter->fn(refname, oid, flags, filter->cb_data);
 }
 
-enum peel_status peel_object(const unsigned char *name, unsigned char *sha1)
+enum peel_status peel_object(const struct object_id *name, struct object_id *oid)
 {
-	struct object *o = lookup_unknown_object(name);
+	struct object *o = lookup_unknown_object(name->hash);
 
 	if (o->type == OBJ_NONE) {
-		int type = sha1_object_info(name, NULL);
+		int type = sha1_object_info(name->hash, NULL);
 		if (type < 0 || !object_as_type(o, type, 0))
 			return PEEL_INVALID;
 	}
@@ -269,7 +269,7 @@ enum peel_status peel_object(const unsigned char *name, unsigned char *sha1)
 	if (!o)
 		return PEEL_INVALID;
 
-	hashcpy(sha1, o->oid.hash);
+	oidcpy(oid, &o->oid);
 	return PEEL_PEELED;
 }
 
@@ -362,7 +362,7 @@ int head_ref_namespaced(each_ref_fn fn, void *cb_data)
 	int flag;
 
 	strbuf_addf(&buf, "%sHEAD", get_git_namespace());
-	if (!read_ref_full(buf.buf, RESOLVE_REF_READING, oid.hash, &flag))
+	if (!read_ref_full(buf.buf, RESOLVE_REF_READING, &oid, &flag))
 		ret = fn(buf.buf, &oid, flag, cb_data);
 	strbuf_release(&buf);
 
@@ -456,15 +456,15 @@ static char *substitute_branch_name(const char **string, int *len)
 	return NULL;
 }
 
-int dwim_ref(const char *str, int len, unsigned char *sha1, char **ref)
+int dwim_ref(const char *str, int len, struct object_id *oid, char **ref)
 {
 	char *last_branch = substitute_branch_name(&str, &len);
-	int   refs_found  = expand_ref(str, len, sha1, ref);
+	int   refs_found  = expand_ref(str, len, oid, ref);
 	free(last_branch);
 	return refs_found;
 }
 
-int expand_ref(const char *str, int len, unsigned char *sha1, char **ref)
+int expand_ref(const char *str, int len, struct object_id *oid, char **ref)
 {
 	const char **p, *r;
 	int refs_found = 0;
@@ -472,11 +472,11 @@ int expand_ref(const char *str, int len, unsigned char *sha1, char **ref)
 
 	*ref = NULL;
 	for (p = ref_rev_parse_rules; *p; p++) {
-		unsigned char sha1_from_ref[20];
-		unsigned char *this_result;
+		struct object_id oid_from_ref;
+		struct object_id *this_result;
 		int flag;
 
-		this_result = refs_found ? sha1_from_ref : sha1;
+		this_result = refs_found ? &oid_from_ref : oid;
 		strbuf_reset(&fullref);
 		strbuf_addf(&fullref, *p, len, str);
 		r = resolve_ref_unsafe(fullref.buf, RESOLVE_REF_READING,
@@ -496,7 +496,7 @@ int expand_ref(const char *str, int len, unsigned char *sha1, char **ref)
 	return refs_found;
 }
 
-int dwim_log(const char *str, int len, unsigned char *sha1, char **log)
+int dwim_log(const char *str, int len, struct object_id *oid, char **log)
 {
 	char *last_branch = substitute_branch_name(&str, &len);
 	const char **p;
@@ -505,13 +505,13 @@ int dwim_log(const char *str, int len, unsigned char *sha1, char **log)
 
 	*log = NULL;
 	for (p = ref_rev_parse_rules; *p; p++) {
-		unsigned char hash[20];
+		struct object_id hash;
 		const char *ref, *it;
 
 		strbuf_reset(&path);
 		strbuf_addf(&path, *p, len, str);
 		ref = resolve_ref_unsafe(path.buf, RESOLVE_REF_READING,
-					 hash, NULL);
+					 &hash, NULL);
 		if (!ref)
 			continue;
 		if (reflog_exists(path.buf))
@@ -522,7 +522,7 @@ int dwim_log(const char *str, int len, unsigned char *sha1, char **log)
 			continue;
 		if (!logs_found++) {
 			*log = xstrdup(it);
-			hashcpy(sha1, hash);
+			oidcpy(oid, &hash);
 		}
 		if (!warn_ambiguous_refs)
 			break;
@@ -574,8 +574,8 @@ long get_files_ref_lock_timeout_ms(void)
 	return timeout_ms;
 }
 
-static int write_pseudoref(const char *pseudoref, const unsigned char *sha1,
-			   const unsigned char *old_sha1, struct strbuf *err)
+static int write_pseudoref(const char *pseudoref, const struct object_id *oid,
+			   const struct object_id *old_oid, struct strbuf *err)
 {
 	const char *filename;
 	int fd;
@@ -583,7 +583,10 @@ static int write_pseudoref(const char *pseudoref, const unsigned char *sha1,
 	struct strbuf buf = STRBUF_INIT;
 	int ret = -1;
 
-	strbuf_addf(&buf, "%s\n", sha1_to_hex(sha1));
+	if (!oid)
+		return 0;
+
+	strbuf_addf(&buf, "%s\n", oid_to_hex(oid));
 
 	filename = git_path("%s", pseudoref);
 	fd = hold_lock_file_for_update_timeout(&lock, filename,
@@ -595,12 +598,12 @@ static int write_pseudoref(const char *pseudoref, const unsigned char *sha1,
 		goto done;
 	}
 
-	if (old_sha1) {
-		unsigned char actual_old_sha1[20];
+	if (old_oid) {
+		struct object_id actual_old_oid;
 
-		if (read_ref(pseudoref, actual_old_sha1))
+		if (read_ref(pseudoref, &actual_old_oid))
 			die("could not read ref '%s'", pseudoref);
-		if (hashcmp(actual_old_sha1, old_sha1)) {
+		if (oidcmp(&actual_old_oid, old_oid)) {
 			strbuf_addf(err, "unexpected sha1 when writing '%s'", pseudoref);
 			rollback_lock_file(&lock);
 			goto done;
@@ -620,25 +623,25 @@ done:
 	return ret;
 }
 
-static int delete_pseudoref(const char *pseudoref, const unsigned char *old_sha1)
+static int delete_pseudoref(const char *pseudoref, const struct object_id *old_oid)
 {
 	static struct lock_file lock;
 	const char *filename;
 
 	filename = git_path("%s", pseudoref);
 
-	if (old_sha1 && !is_null_sha1(old_sha1)) {
+	if (old_oid && !is_null_oid(old_oid)) {
 		int fd;
-		unsigned char actual_old_sha1[20];
+		struct object_id actual_old_oid;
 
 		fd = hold_lock_file_for_update_timeout(
 				&lock, filename, LOCK_DIE_ON_ERROR,
 				get_files_ref_lock_timeout_ms());
 		if (fd < 0)
 			die_errno(_("Could not open '%s' for writing"), filename);
-		if (read_ref(pseudoref, actual_old_sha1))
+		if (read_ref(pseudoref, &actual_old_oid))
 			die("could not read ref '%s'", pseudoref);
-		if (hashcmp(actual_old_sha1, old_sha1)) {
+		if (oidcmp(&actual_old_oid, old_oid)) {
 			warning("Unexpected sha1 when deleting %s", pseudoref);
 			rollback_lock_file(&lock);
 			return -1;
@@ -655,7 +658,7 @@ static int delete_pseudoref(const char *pseudoref, const unsigned char *old_sha1
 
 int refs_delete_ref(struct ref_store *refs, const char *msg,
 		    const char *refname,
-		    const unsigned char *old_sha1,
+		    const struct object_id *old_oid,
 		    unsigned int flags)
 {
 	struct ref_transaction *transaction;
@@ -663,12 +666,12 @@ int refs_delete_ref(struct ref_store *refs, const char *msg,
 
 	if (ref_type(refname) == REF_TYPE_PSEUDOREF) {
 		assert(refs == get_main_ref_store());
-		return delete_pseudoref(refname, old_sha1);
+		return delete_pseudoref(refname, old_oid);
 	}
 
 	transaction = ref_store_transaction_begin(refs, &err);
 	if (!transaction ||
-	    ref_transaction_delete(transaction, refname, old_sha1,
+	    ref_transaction_delete(transaction, refname, old_oid,
 				   flags, msg, &err) ||
 	    ref_transaction_commit(transaction, &err)) {
 		error("%s", err.buf);
@@ -682,10 +685,10 @@ int refs_delete_ref(struct ref_store *refs, const char *msg,
 }
 
 int delete_ref(const char *msg, const char *refname,
-	       const unsigned char *old_sha1, unsigned int flags)
+	       const struct object_id *old_oid, unsigned int flags)
 {
 	return refs_delete_ref(get_main_ref_store(), msg, refname,
-			       old_sha1, flags);
+			       old_oid, flags);
 }
 
 int copy_reflog_msg(char *buf, const char *msg)
@@ -734,11 +737,11 @@ struct read_ref_at_cb {
 	timestamp_t at_time;
 	int cnt;
 	int reccnt;
-	unsigned char *sha1;
+	struct object_id *oid;
 	int found_it;
 
-	unsigned char osha1[20];
-	unsigned char nsha1[20];
+	struct object_id ooid;
+	struct object_id noid;
 	int tz;
 	timestamp_t date;
 	char **msg;
@@ -770,25 +773,25 @@ static int read_ref_at_ent(struct object_id *ooid, struct object_id *noid,
 		 * we have not yet updated cb->[n|o]sha1 so they still
 		 * hold the values for the previous record.
 		 */
-		if (!is_null_sha1(cb->osha1)) {
-			hashcpy(cb->sha1, noid->hash);
-			if (hashcmp(cb->osha1, noid->hash))
+		if (!is_null_oid(&cb->ooid)) {
+			oidcpy(cb->oid, noid);
+			if (oidcmp(&cb->ooid, noid))
 				warning("Log for ref %s has gap after %s.",
 					cb->refname, show_date(cb->date, cb->tz, DATE_MODE(RFC2822)));
 		}
 		else if (cb->date == cb->at_time)
-			hashcpy(cb->sha1, noid->hash);
-		else if (hashcmp(noid->hash, cb->sha1))
+			oidcpy(cb->oid, noid);
+		else if (oidcmp(noid, cb->oid))
 			warning("Log for ref %s unexpectedly ended on %s.",
 				cb->refname, show_date(cb->date, cb->tz,
 						       DATE_MODE(RFC2822)));
-		hashcpy(cb->osha1, ooid->hash);
-		hashcpy(cb->nsha1, noid->hash);
+		oidcpy(&cb->ooid, ooid);
+		oidcpy(&cb->noid, noid);
 		cb->found_it = 1;
 		return 1;
 	}
-	hashcpy(cb->osha1, ooid->hash);
-	hashcpy(cb->nsha1, noid->hash);
+	oidcpy(&cb->ooid, ooid);
+	oidcpy(&cb->noid, noid);
 	if (cb->cnt > 0)
 		cb->cnt--;
 	return 0;
@@ -808,15 +811,15 @@ static int read_ref_at_ent_oldest(struct object_id *ooid, struct object_id *noid
 		*cb->cutoff_tz = tz;
 	if (cb->cutoff_cnt)
 		*cb->cutoff_cnt = cb->reccnt;
-	hashcpy(cb->sha1, ooid->hash);
-	if (is_null_sha1(cb->sha1))
-		hashcpy(cb->sha1, noid->hash);
+	oidcpy(cb->oid, ooid);
+	if (is_null_oid(cb->oid))
+		oidcpy(cb->oid, noid);
 	/* We just want the first entry */
 	return 1;
 }
 
 int read_ref_at(const char *refname, unsigned int flags, timestamp_t at_time, int cnt,
-		unsigned char *sha1, char **msg,
+		struct object_id *oid, char **msg,
 		timestamp_t *cutoff_time, int *cutoff_tz, int *cutoff_cnt)
 {
 	struct read_ref_at_cb cb;
@@ -829,7 +832,7 @@ int read_ref_at(const char *refname, unsigned int flags, timestamp_t at_time, in
 	cb.cutoff_time = cutoff_time;
 	cb.cutoff_tz = cutoff_tz;
 	cb.cutoff_cnt = cutoff_cnt;
-	cb.sha1 = sha1;
+	cb.oid = oid;
 
 	for_each_reflog_ent_reverse(refname, read_ref_at_ent, &cb);
 
@@ -894,8 +897,8 @@ void ref_transaction_free(struct ref_transaction *transaction)
 struct ref_update *ref_transaction_add_update(
 		struct ref_transaction *transaction,
 		const char *refname, unsigned int flags,
-		const unsigned char *new_sha1,
-		const unsigned char *old_sha1,
+		const struct object_id *new_oid,
+		const struct object_id *old_oid,
 		const char *msg)
 {
 	struct ref_update *update;
@@ -913,23 +916,23 @@ struct ref_update *ref_transaction_add_update(
 	update->flags = flags;
 
 	if (flags & REF_HAVE_NEW)
-		hashcpy(update->new_oid.hash, new_sha1);
+		oidcpy(&update->new_oid, new_oid);
 	if (flags & REF_HAVE_OLD)
-		hashcpy(update->old_oid.hash, old_sha1);
+		oidcpy(&update->old_oid, old_oid);
 	update->msg = xstrdup_or_null(msg);
 	return update;
 }
 
 int ref_transaction_update(struct ref_transaction *transaction,
 			   const char *refname,
-			   const unsigned char *new_sha1,
-			   const unsigned char *old_sha1,
+			   const struct object_id *new_oid,
+			   const struct object_id *old_oid,
 			   unsigned int flags, const char *msg,
 			   struct strbuf *err)
 {
 	assert(err);
 
-	if ((new_sha1 && !is_null_sha1(new_sha1)) ?
+	if ((new_oid && !is_null_oid(new_oid)) ?
 	    check_refname_format(refname, REFNAME_ALLOW_ONELEVEL) :
 	    !refname_is_safe(refname)) {
 		strbuf_addf(err, "refusing to update ref with bad name '%s'",
@@ -939,62 +942,54 @@ int ref_transaction_update(struct ref_transaction *transaction,
 
 	flags &= REF_TRANSACTION_UPDATE_ALLOWED_FLAGS;
 
-	flags |= (new_sha1 ? REF_HAVE_NEW : 0) | (old_sha1 ? REF_HAVE_OLD : 0);
+	flags |= (new_oid ? REF_HAVE_NEW : 0) | (old_oid ? REF_HAVE_OLD : 0);
 
 	ref_transaction_add_update(transaction, refname, flags,
-				   new_sha1, old_sha1, msg);
+				   new_oid, old_oid, msg);
 	return 0;
 }
 
 int ref_transaction_create(struct ref_transaction *transaction,
 			   const char *refname,
-			   const unsigned char *new_sha1,
+			   const struct object_id *new_oid,
 			   unsigned int flags, const char *msg,
 			   struct strbuf *err)
 {
-	if (!new_sha1 || is_null_sha1(new_sha1))
-		die("BUG: create called without valid new_sha1");
-	return ref_transaction_update(transaction, refname, new_sha1,
-				      null_sha1, flags, msg, err);
+	if (!new_oid || is_null_oid(new_oid))
+		die("BUG: create called without valid new_oid");
+	return ref_transaction_update(transaction, refname, new_oid,
+				      &null_oid, flags, msg, err);
 }
 
 int ref_transaction_delete(struct ref_transaction *transaction,
 			   const char *refname,
-			   const unsigned char *old_sha1,
+			   const struct object_id *old_oid,
 			   unsigned int flags, const char *msg,
 			   struct strbuf *err)
 {
-	if (old_sha1 && is_null_sha1(old_sha1))
-		die("BUG: delete called with old_sha1 set to zeros");
+	if (old_oid && is_null_oid(old_oid))
+		die("BUG: delete called with old_oid set to zeros");
 	return ref_transaction_update(transaction, refname,
-				      null_sha1, old_sha1,
+				      &null_oid, old_oid,
 				      flags, msg, err);
 }
 
 int ref_transaction_verify(struct ref_transaction *transaction,
 			   const char *refname,
-			   const unsigned char *old_sha1,
+			   const struct object_id *old_oid,
 			   unsigned int flags,
 			   struct strbuf *err)
 {
-	if (!old_sha1)
-		die("BUG: verify called with old_sha1 set to NULL");
+	if (!old_oid)
+		die("BUG: verify called with old_oid set to NULL");
 	return ref_transaction_update(transaction, refname,
-				      NULL, old_sha1,
+				      NULL, old_oid,
 				      flags, NULL, err);
 }
 
-int update_ref_oid(const char *msg, const char *refname,
-	       const struct object_id *new_oid, const struct object_id *old_oid,
-	       unsigned int flags, enum action_on_err onerr)
-{
-	return update_ref(msg, refname, new_oid ? new_oid->hash : NULL,
-		old_oid ? old_oid->hash : NULL, flags, onerr);
-}
-
 int refs_update_ref(struct ref_store *refs, const char *msg,
-		    const char *refname, const unsigned char *new_sha1,
-		    const unsigned char *old_sha1, unsigned int flags,
+		    const char *refname, const struct object_id *new_oid,
+		    const struct object_id *old_oid, unsigned int flags,
 		    enum action_on_err onerr)
 {
 	struct ref_transaction *t = NULL;
@@ -1003,11 +998,11 @@ int refs_update_ref(struct ref_store *refs, const char *msg,
 
 	if (ref_type(refname) == REF_TYPE_PSEUDOREF) {
 		assert(refs == get_main_ref_store());
-		ret = write_pseudoref(refname, new_sha1, old_sha1, &err);
+		ret = write_pseudoref(refname, new_oid, old_oid, &err);
 	} else {
 		t = ref_store_transaction_begin(refs, &err);
 		if (!t ||
-		    ref_transaction_update(t, refname, new_sha1, old_sha1,
+		    ref_transaction_update(t, refname, new_oid, old_oid,
 					   flags, msg, &err) ||
 		    ref_transaction_commit(t, &err)) {
 			ret = 1;
@@ -1037,12 +1032,12 @@ int refs_update_ref(struct ref_store *refs, const char *msg,
 }
 
 int update_ref(const char *msg, const char *refname,
-	       const unsigned char *new_sha1,
-	       const unsigned char *old_sha1,
+	       const struct object_id *new_oid,
+	       const struct object_id *old_oid,
 	       unsigned int flags, enum action_on_err onerr)
 {
-	return refs_update_ref(get_main_ref_store(), msg, refname, new_sha1,
-			       old_sha1, flags, onerr);
+	return refs_update_ref(get_main_ref_store(), msg, refname, new_oid,
+			       old_oid, flags, onerr);
 }
 
 char *shorten_unambiguous_ref(const char *refname, int strict)
@@ -1254,7 +1249,7 @@ int refs_head_ref(struct ref_store *refs, each_ref_fn fn, void *cb_data)
 	int flag;
 
 	if (!refs_read_ref_full(refs, "HEAD", RESOLVE_REF_READING,
-				oid.hash, &flag))
+				&oid, &flag))
 		return fn("HEAD", &oid, flag, cb_data);
 
 	return 0;
@@ -1387,25 +1382,25 @@ int for_each_rawref(each_ref_fn fn, void *cb_data)
 }
 
 int refs_read_raw_ref(struct ref_store *ref_store,
-		      const char *refname, unsigned char *sha1,
+		      const char *refname, struct object_id *oid,
 		      struct strbuf *referent, unsigned int *type)
 {
-	return ref_store->be->read_raw_ref(ref_store, refname, sha1, referent, type);
+	return ref_store->be->read_raw_ref(ref_store, refname, oid, referent, type);
 }
 
 /* This function needs to return a meaningful errno on failure */
 const char *refs_resolve_ref_unsafe(struct ref_store *refs,
 				    const char *refname,
 				    int resolve_flags,
-				    unsigned char *sha1, int *flags)
+				    struct object_id *oid, int *flags)
 {
 	static struct strbuf sb_refname = STRBUF_INIT;
 	struct object_id unused_oid;
 	int unused_flags;
 	int symref_count;
 
-	if (!sha1)
-		sha1 = unused_oid.hash;
+	if (!oid)
+		oid = &unused_oid;
 	if (!flags)
 		flags = &unused_flags;
 
@@ -1433,7 +1428,7 @@ const char *refs_resolve_ref_unsafe(struct ref_store *refs,
 		unsigned int read_flags = 0;
 
 		if (refs_read_raw_ref(refs, refname,
-				      sha1, &sb_refname, &read_flags)) {
+				      oid, &sb_refname, &read_flags)) {
 			*flags |= read_flags;
 
 			/* In reading mode, refs must eventually resolve */
@@ -1450,7 +1445,7 @@ const char *refs_resolve_ref_unsafe(struct ref_store *refs,
 			    errno != ENOTDIR)
 				return NULL;
 
-			hashclr(sha1);
+			oidclr(oid);
 			if (*flags & REF_BAD_NAME)
 				*flags |= REF_ISBROKEN;
 			return refname;
@@ -1460,7 +1455,7 @@ const char *refs_resolve_ref_unsafe(struct ref_store *refs,
 
 		if (!(read_flags & REF_ISSYMREF)) {
 			if (*flags & REF_BAD_NAME) {
-				hashclr(sha1);
+				oidclr(oid);
 				*flags |= REF_ISBROKEN;
 			}
 			return refname;
@@ -1468,7 +1463,7 @@ const char *refs_resolve_ref_unsafe(struct ref_store *refs,
 
 		refname = sb_refname.buf;
 		if (resolve_flags & RESOLVE_REF_NO_RECURSE) {
-			hashclr(sha1);
+			oidclr(oid);
 			return refname;
 		}
 		if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL)) {
@@ -1495,14 +1490,14 @@ int refs_init_db(struct strbuf *err)
 }
 
 const char *resolve_ref_unsafe(const char *refname, int resolve_flags,
-			       unsigned char *sha1, int *flags)
+			       struct object_id *oid, int *flags)
 {
 	return refs_resolve_ref_unsafe(get_main_ref_store(), refname,
-				       resolve_flags, sha1, flags);
+				       resolve_flags, oid, flags);
 }
 
 int resolve_gitlink_ref(const char *submodule, const char *refname,
-			unsigned char *sha1)
+			struct object_id *oid)
 {
 	struct ref_store *refs;
 	int flags;
@@ -1512,8 +1507,8 @@ int resolve_gitlink_ref(const char *submodule, const char *refname,
 	if (!refs)
 		return -1;
 
-	if (!refs_resolve_ref_unsafe(refs, refname, 0, sha1, &flags) ||
-	    is_null_sha1(sha1))
+	if (!refs_resolve_ref_unsafe(refs, refname, 0, oid, &flags) ||
+	    is_null_oid(oid))
 		return -1;
 	return 0;
 }
@@ -1701,30 +1696,30 @@ int refs_pack_refs(struct ref_store *refs, unsigned int flags)
 }
 
 int refs_peel_ref(struct ref_store *refs, const char *refname,
-		  unsigned char *sha1)
+		  struct object_id *oid)
 {
 	int flag;
-	unsigned char base[20];
+	struct object_id base;
 
 	if (current_ref_iter && current_ref_iter->refname == refname) {
 		struct object_id peeled;
 
 		if (ref_iterator_peel(current_ref_iter, &peeled))
 			return -1;
-		hashcpy(sha1, peeled.hash);
+		oidcpy(oid, &peeled);
 		return 0;
 	}
 
 	if (refs_read_ref_full(refs, refname,
-			       RESOLVE_REF_READING, base, &flag))
+			       RESOLVE_REF_READING, &base, &flag))
 		return -1;
 
-	return peel_object(base, sha1);
+	return peel_object(&base, oid);
 }
 
-int peel_ref(const char *refname, unsigned char *sha1)
+int peel_ref(const char *refname, struct object_id *oid)
 {
-	return refs_peel_ref(get_main_ref_store(), refname, sha1);
+	return refs_peel_ref(get_main_ref_store(), refname, oid);
 }
 
 int refs_create_symref(struct ref_store *refs,
@@ -1884,7 +1879,7 @@ int refs_verify_refname_available(struct ref_store *refs,
 		if (skip && string_list_has_string(skip, dirname.buf))
 			continue;
 
-		if (!refs_read_raw_ref(refs, dirname.buf, oid.hash, &referent, &type)) {
+		if (!refs_read_raw_ref(refs, dirname.buf, &oid, &referent, &type)) {
 			strbuf_addf(err, "'%s' exists; cannot create '%s'",
 				    dirname.buf, refname);
 			goto cleanup;
@@ -2014,19 +2009,19 @@ int delete_reflog(const char *refname)
 }
 
 int refs_reflog_expire(struct ref_store *refs,
-		       const char *refname, const unsigned char *sha1,
+		       const char *refname, const struct object_id *oid,
 		       unsigned int flags,
 		       reflog_expiry_prepare_fn prepare_fn,
 		       reflog_expiry_should_prune_fn should_prune_fn,
 		       reflog_expiry_cleanup_fn cleanup_fn,
 		       void *policy_cb_data)
 {
-	return refs->be->reflog_expire(refs, refname, sha1, flags,
+	return refs->be->reflog_expire(refs, refname, oid, flags,
 				       prepare_fn, should_prune_fn,
 				       cleanup_fn, policy_cb_data);
 }
 
-int reflog_expire(const char *refname, const unsigned char *sha1,
+int reflog_expire(const char *refname, const struct object_id *oid,
 		  unsigned int flags,
 		  reflog_expiry_prepare_fn prepare_fn,
 		  reflog_expiry_should_prune_fn should_prune_fn,
@@ -2034,7 +2029,7 @@ int reflog_expire(const char *refname, const unsigned char *sha1,
 		  void *policy_cb_data)
 {
 	return refs_reflog_expire(get_main_ref_store(),
-				  refname, sha1, flags,
+				  refname, oid, flags,
 				  prepare_fn, should_prune_fn,
 				  cleanup_fn, policy_cb_data);
 }
diff --git a/refs.h b/refs.h
index a02b628c8f..15fd419c7d 100644
--- a/refs.h
+++ b/refs.h
@@ -14,22 +14,22 @@ struct worktree;
  * at the resolved object name.  The return value, if not NULL, is a
  * pointer into either a static buffer or the input ref.
  *
- * If sha1 is non-NULL, store the referred-to object's name in it.
+ * If oid is non-NULL, store the referred-to object's name in it.
  *
  * If the reference cannot be resolved to an object, the behavior
  * depends on the RESOLVE_REF_READING flag:
  *
  * - If RESOLVE_REF_READING is set, return NULL.
  *
- * - If RESOLVE_REF_READING is not set, clear sha1 and return the name of
+ * - If RESOLVE_REF_READING is not set, clear oid and return the name of
  *   the last reference name in the chain, which will either be a non-symbolic
  *   reference or an undefined reference.  If this is a prelude to
  *   "writing" to the ref, the return value is the name of the ref
  *   that will actually be created or changed.
  *
  * If the RESOLVE_REF_NO_RECURSE flag is passed, only resolves one
- * level of symbolic reference.  The value stored in sha1 for a symbolic
- * reference will always be null_sha1 in this case, and the return
+ * level of symbolic reference.  The value stored in oid for a symbolic
+ * reference will always be null_oid in this case, and the return
  * value is the reference that the symref refers to directly.
  *
  * If flags is non-NULL, set the value that it points to the
@@ -46,7 +46,7 @@ struct worktree;
  *
  * RESOLVE_REF_ALLOW_BAD_NAME allows resolving refs even when their
  * name is invalid according to git-check-ref-format(1).  If the name
- * is bad then the value stored in sha1 will be null_sha1 and the two
+ * is bad then the value stored in oid will be null_oid and the two
  * flags REF_ISBROKEN and REF_BAD_NAME will be set.
  *
  * Even with RESOLVE_REF_ALLOW_BAD_NAME, names that escape the refs/
@@ -62,22 +62,22 @@ struct worktree;
 const char *refs_resolve_ref_unsafe(struct ref_store *refs,
 				    const char *refname,
 				    int resolve_flags,
-				    unsigned char *sha1,
+				    struct object_id *oid,
 				    int *flags);
 const char *resolve_ref_unsafe(const char *refname, int resolve_flags,
-			       unsigned char *sha1, int *flags);
+			       struct object_id *oid, int *flags);
 
 char *refs_resolve_refdup(struct ref_store *refs,
 			  const char *refname, int resolve_flags,
-			  unsigned char *sha1, int *flags);
+			  struct object_id *oid, int *flags);
 char *resolve_refdup(const char *refname, int resolve_flags,
-		     unsigned char *sha1, int *flags);
+		     struct object_id *oid, int *flags);
 
 int refs_read_ref_full(struct ref_store *refs, const char *refname,
-		       int resolve_flags, unsigned char *sha1, int *flags);
+		       int resolve_flags, struct object_id *oid, int *flags);
 int read_ref_full(const char *refname, int resolve_flags,
-		  unsigned char *sha1, int *flags);
-int read_ref(const char *refname, unsigned char *sha1);
+		  struct object_id *oid, int *flags);
+int read_ref(const char *refname, struct object_id *oid);
 
 /*
  * Return 0 if a reference named refname could be created without
@@ -114,14 +114,14 @@ extern int refs_init_db(struct strbuf *err);
 /*
  * If refname is a non-symbolic reference that refers to a tag object,
  * and the tag can be (recursively) dereferenced to a non-tag object,
- * store the SHA1 of the referred-to object to sha1 and return 0.  If
- * any of these conditions are not met, return a non-zero value.
+ * store the object ID of the referred-to object to oid and return 0.
+ * If any of these conditions are not met, return a non-zero value.
  * Symbolic references are considered unpeelable, even if they
  * ultimately resolve to a peelable tag.
  */
 int refs_peel_ref(struct ref_store *refs, const char *refname,
-		  unsigned char *sha1);
-int peel_ref(const char *refname, unsigned char *sha1);
+		  struct object_id *oid);
+int peel_ref(const char *refname, struct object_id *oid);
 
 /**
  * Resolve refname in the nested "gitlink" repository in the specified
@@ -130,7 +130,7 @@ int peel_ref(const char *refname, unsigned char *sha1);
  * otherwise, return a non-zero value.
  */
 int resolve_gitlink_ref(const char *submodule, const char *refname,
-			unsigned char *sha1);
+			struct object_id *oid);
 
 /*
  * Return true iff abbrev_name is a possible abbreviation for
@@ -139,9 +139,9 @@ int resolve_gitlink_ref(const char *submodule, const char *refname,
  */
 int refname_match(const char *abbrev_name, const char *full_name);
 
-int expand_ref(const char *str, int len, unsigned char *sha1, char **ref);
-int dwim_ref(const char *str, int len, unsigned char *sha1, char **ref);
-int dwim_log(const char *str, int len, unsigned char *sha1, char **ref);
+int expand_ref(const char *str, int len, struct object_id *oid, char **ref);
+int dwim_ref(const char *str, int len, struct object_id *oid, char **ref);
+int dwim_log(const char *str, int len, struct object_id *oid, char **ref);
 
 /*
  * A ref_transaction represents a collection of reference updates that
@@ -363,7 +363,7 @@ int safe_create_reflog(const char *refname, int force_create, struct strbuf *err
 /** Reads log for the value of ref during at_time. **/
 int read_ref_at(const char *refname, unsigned int flags,
 		timestamp_t at_time, int cnt,
-		unsigned char *sha1, char **msg,
+		struct object_id *oid, char **msg,
 		timestamp_t *cutoff_time, int *cutoff_tz, int *cutoff_cnt);
 
 /** Check if a particular reflog exists */
@@ -371,19 +371,19 @@ int refs_reflog_exists(struct ref_store *refs, const char *refname);
 int reflog_exists(const char *refname);
 
 /*
- * Delete the specified reference. If old_sha1 is non-NULL, then
+ * Delete the specified reference. If old_oid is non-NULL, then
  * verify that the current value of the reference is old_sha1 before
- * deleting it. If old_sha1 is NULL, delete the reference if it
- * exists, regardless of its old value. It is an error for old_sha1 to
- * be NULL_SHA1. msg and flags are passed through to
+ * deleting it. If old_oid is NULL, delete the reference if it
+ * exists, regardless of its old value. It is an error for old_oid to
+ * be null_oid. msg and flags are passed through to
  * ref_transaction_delete().
  */
 int refs_delete_ref(struct ref_store *refs, const char *msg,
 		    const char *refname,
-		    const unsigned char *old_sha1,
+		    const struct object_id *old_oid,
 		    unsigned int flags);
 int delete_ref(const char *msg, const char *refname,
-	       const unsigned char *old_sha1, unsigned int flags);
+	       const struct object_id *old_oid, unsigned int flags);
 
 /*
  * Delete the specified references. If there are any problems, emit
@@ -511,14 +511,14 @@ struct ref_transaction *ref_transaction_begin(struct strbuf *err);
  */
 
 /*
- * Add a reference update to transaction. new_sha1 is the value that
- * the reference should have after the update, or null_sha1 if it
- * should be deleted. If new_sha1 is NULL, then the reference is not
- * changed at all. old_sha1 is the value that the reference must have
- * before the update, or null_sha1 if it must not have existed
+ * Add a reference update to transaction. new_oid is the value that
+ * the reference should have after the update, or null_oid if it
+ * should be deleted. If new_oid is NULL, then the reference is not
+ * changed at all. old_oid is the value that the reference must have
+ * before the update, or null_oid if it must not have existed
  * beforehand. The old value is checked after the lock is taken to
- * prevent races. If the old value doesn't agree with old_sha1, the
- * whole transaction fails. If old_sha1 is NULL, then the previous
+ * prevent races. If the old value doesn't agree with old_oid, the
+ * whole transaction fails. If old_oid is NULL, then the previous
  * value is not checked.
  *
  * See the above comment "Reference transaction updates" for more
@@ -526,15 +526,15 @@ struct ref_transaction *ref_transaction_begin(struct strbuf *err);
  */
 int ref_transaction_update(struct ref_transaction *transaction,
 			   const char *refname,
-			   const unsigned char *new_sha1,
-			   const unsigned char *old_sha1,
+			   const struct object_id *new_oid,
+			   const struct object_id *old_oid,
 			   unsigned int flags, const char *msg,
 			   struct strbuf *err);
 
 /*
- * Add a reference creation to transaction. new_sha1 is the value that
+ * Add a reference creation to transaction. new_oid is the value that
  * the reference should have after the update; it must not be
- * null_sha1. It is verified that the reference does not exist
+ * null_oid. It is verified that the reference does not exist
  * already.
  *
  * See the above comment "Reference transaction updates" for more
@@ -542,35 +542,35 @@ int ref_transaction_update(struct ref_transaction *transaction,
  */
 int ref_transaction_create(struct ref_transaction *transaction,
 			   const char *refname,
-			   const unsigned char *new_sha1,
+			   const struct object_id *new_oid,
 			   unsigned int flags, const char *msg,
 			   struct strbuf *err);
 
 /*
- * Add a reference deletion to transaction. If old_sha1 is non-NULL,
+ * Add a reference deletion to transaction. If old_oid is non-NULL,
  * then it holds the value that the reference should have had before
- * the update (which must not be null_sha1).
+ * the update (which must not be null_oid).
  *
  * See the above comment "Reference transaction updates" for more
  * information.
  */
 int ref_transaction_delete(struct ref_transaction *transaction,
 			   const char *refname,
-			   const unsigned char *old_sha1,
+			   const struct object_id *old_oid,
 			   unsigned int flags, const char *msg,
 			   struct strbuf *err);
 
 /*
- * Verify, within a transaction, that refname has the value old_sha1,
- * or, if old_sha1 is null_sha1, then verify that the reference
- * doesn't exist. old_sha1 must be non-NULL.
+ * Verify, within a transaction, that refname has the value old_oid,
+ * or, if old_oid is null_oid, then verify that the reference
+ * doesn't exist. old_oid must be non-NULL.
  *
  * See the above comment "Reference transaction updates" for more
  * information.
  */
 int ref_transaction_verify(struct ref_transaction *transaction,
 			   const char *refname,
-			   const unsigned char *old_sha1,
+			   const struct object_id *old_oid,
 			   unsigned int flags,
 			   struct strbuf *err);
 
@@ -643,12 +643,9 @@ void ref_transaction_free(struct ref_transaction *transaction);
  * argument.
  */
 int refs_update_ref(struct ref_store *refs, const char *msg, const char *refname,
-		    const unsigned char *new_sha1, const unsigned char *old_sha1,
+		    const struct object_id *new_oid, const struct object_id *old_oid,
 		    unsigned int flags, enum action_on_err onerr);
 int update_ref(const char *msg, const char *refname,
-	       const unsigned char *new_sha1, const unsigned char *old_sha1,
-	       unsigned int flags, enum action_on_err onerr);
-int update_ref_oid(const char *msg, const char *refname,
 	       const struct object_id *new_oid, const struct object_id *old_oid,
 	       unsigned int flags, enum action_on_err onerr);
 
@@ -706,20 +703,20 @@ typedef int reflog_expiry_should_prune_fn(struct object_id *ooid,
 typedef void reflog_expiry_cleanup_fn(void *cb_data);
 
 /*
- * Expire reflog entries for the specified reference. sha1 is the old
+ * Expire reflog entries for the specified reference. oid is the old
  * value of the reference. flags is a combination of the constants in
  * enum expire_reflog_flags. The three function pointers are described
  * above. On success, return zero.
  */
 int refs_reflog_expire(struct ref_store *refs,
 		       const char *refname,
-		       const unsigned char *sha1,
+		       const struct object_id *oid,
 		       unsigned int flags,
 		       reflog_expiry_prepare_fn prepare_fn,
 		       reflog_expiry_should_prune_fn should_prune_fn,
 		       reflog_expiry_cleanup_fn cleanup_fn,
 		       void *policy_cb_data);
-int reflog_expire(const char *refname, const unsigned char *sha1,
+int reflog_expire(const char *refname, const struct object_id *oid,
 		  unsigned int flags,
 		  reflog_expiry_prepare_fn prepare_fn,
 		  reflog_expiry_should_prune_fn should_prune_fn,
diff --git a/refs/files-backend.c b/refs/files-backend.c
index 8cc1e07fdb..2bd54e11ae 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -189,7 +189,7 @@ static void loose_fill_ref_dir(struct ref_store *ref_store,
 			if (!refs_resolve_ref_unsafe(&refs->base,
 						     refname.buf,
 						     RESOLVE_REF_READING,
-						     oid.hash, &flag)) {
+						     &oid, &flag)) {
 				oidclr(&oid);
 				flag |= REF_ISBROKEN;
 			} else if (is_null_oid(&oid)) {
@@ -261,7 +261,7 @@ static struct ref_cache *get_loose_ref_cache(struct files_ref_store *refs)
 }
 
 static int files_read_raw_ref(struct ref_store *ref_store,
-			      const char *refname, unsigned char *sha1,
+			      const char *refname, struct object_id *oid,
 			      struct strbuf *referent, unsigned int *type)
 {
 	struct files_ref_store *refs =
@@ -270,6 +270,7 @@ static int files_read_raw_ref(struct ref_store *ref_store,
 	struct strbuf sb_path = STRBUF_INIT;
 	const char *path;
 	const char *buf;
+	const char *p;
 	struct stat st;
 	int fd;
 	int ret = -1;
@@ -304,7 +305,7 @@ stat_ref:
 		if (errno != ENOENT)
 			goto out;
 		if (refs_read_raw_ref(refs->packed_ref_store, refname,
-				      sha1, referent, type)) {
+				      oid, referent, type)) {
 			errno = ENOENT;
 			goto out;
 		}
@@ -344,7 +345,7 @@ stat_ref:
 		 * packed ref:
 		 */
 		if (refs_read_raw_ref(refs->packed_ref_store, refname,
-				      sha1, referent, type)) {
+				      oid, referent, type)) {
 			errno = EISDIR;
 			goto out;
 		}
@@ -390,8 +391,8 @@ stat_ref:
 	 * Please note that FETCH_HEAD has additional
 	 * data after the sha.
 	 */
-	if (get_sha1_hex(buf, sha1) ||
-	    (buf[40] != '\0' && !isspace(buf[40]))) {
+	if (parse_oid_hex(buf, oid, &p) ||
+	    (*p != '\0' && !isspace(*p))) {
 		*type |= REF_ISBROKEN;
 		errno = EINVAL;
 		goto out;
@@ -545,7 +546,7 @@ retry:
 	 */
 
 	if (files_read_raw_ref(&refs->base, refname,
-			       lock->old_oid.hash, referent, type)) {
+			       &lock->old_oid, referent, type)) {
 		if (errno == ENOENT) {
 			if (mustexist) {
 				/* Garden variety missing reference. */
@@ -769,21 +770,21 @@ static struct ref_iterator *files_ref_iterator_begin(
 }
 
 /*
- * Verify that the reference locked by lock has the value old_sha1.
- * Fail if the reference doesn't exist and mustexist is set. Return 0
- * on success. On error, write an error message to err, set errno, and
- * return a negative value.
+ * Verify that the reference locked by lock has the value old_oid
+ * (unless it is NULL).  Fail if the reference doesn't exist and
+ * mustexist is set. Return 0 on success. On error, write an error
+ * message to err, set errno, and return a negative value.
  */
 static int verify_lock(struct ref_store *ref_store, struct ref_lock *lock,
-		       const unsigned char *old_sha1, int mustexist,
+		       const struct object_id *old_oid, int mustexist,
 		       struct strbuf *err)
 {
 	assert(err);
 
 	if (refs_read_ref_full(ref_store, lock->ref_name,
 			       mustexist ? RESOLVE_REF_READING : 0,
-			       lock->old_oid.hash, NULL)) {
-		if (old_sha1) {
+			       &lock->old_oid, NULL)) {
+		if (old_oid) {
 			int save_errno = errno;
 			strbuf_addf(err, "can't verify ref '%s'", lock->ref_name);
 			errno = save_errno;
@@ -793,11 +794,11 @@ static int verify_lock(struct ref_store *ref_store, struct ref_lock *lock,
 			return 0;
 		}
 	}
-	if (old_sha1 && hashcmp(lock->old_oid.hash, old_sha1)) {
+	if (old_oid && oidcmp(&lock->old_oid, old_oid)) {
 		strbuf_addf(err, "ref '%s' is at %s but expected %s",
 			    lock->ref_name,
 			    oid_to_hex(&lock->old_oid),
-			    sha1_to_hex(old_sha1));
+			    oid_to_hex(old_oid));
 		errno = EBUSY;
 		return -1;
 	}
@@ -827,22 +828,22 @@ static int create_reflock(const char *path, void *cb)
  * Locks a ref returning the lock on success and NULL on failure.
  * On failure errno is set to something meaningful.
  */
-static struct ref_lock *lock_ref_sha1_basic(struct files_ref_store *refs,
-					    const char *refname,
-					    const unsigned char *old_sha1,
-					    const struct string_list *extras,
-					    const struct string_list *skip,
-					    unsigned int flags, int *type,
-					    struct strbuf *err)
+static struct ref_lock *lock_ref_oid_basic(struct files_ref_store *refs,
+					   const char *refname,
+					   const struct object_id *old_oid,
+					   const struct string_list *extras,
+					   const struct string_list *skip,
+					   unsigned int flags, int *type,
+					   struct strbuf *err)
 {
 	struct strbuf ref_file = STRBUF_INIT;
 	struct ref_lock *lock;
 	int last_errno = 0;
-	int mustexist = (old_sha1 && !is_null_sha1(old_sha1));
+	int mustexist = (old_oid && !is_null_oid(old_oid));
 	int resolve_flags = RESOLVE_REF_NO_RECURSE;
 	int resolved;
 
-	files_assert_main_repository(refs, "lock_ref_sha1_basic");
+	files_assert_main_repository(refs, "lock_ref_oid_basic");
 	assert(err);
 
 	lock = xcalloc(1, sizeof(struct ref_lock));
@@ -855,7 +856,7 @@ static struct ref_lock *lock_ref_sha1_basic(struct files_ref_store *refs,
 	files_ref_path(refs, &ref_file, refname);
 	resolved = !!refs_resolve_ref_unsafe(&refs->base,
 					     refname, resolve_flags,
-					     lock->old_oid.hash, type);
+					     &lock->old_oid, type);
 	if (!resolved && errno == EISDIR) {
 		/*
 		 * we are trying to lock foo but we used to
@@ -874,7 +875,7 @@ static struct ref_lock *lock_ref_sha1_basic(struct files_ref_store *refs,
 		}
 		resolved = !!refs_resolve_ref_unsafe(&refs->base,
 						     refname, resolve_flags,
-						     lock->old_oid.hash, type);
+						     &lock->old_oid, type);
 	}
 	if (!resolved) {
 		last_errno = errno;
@@ -908,7 +909,7 @@ static struct ref_lock *lock_ref_sha1_basic(struct files_ref_store *refs,
 		goto error_return;
 	}
 
-	if (verify_lock(&refs->base, lock, old_sha1, mustexist, err)) {
+	if (verify_lock(&refs->base, lock, old_oid, mustexist, err)) {
 		last_errno = errno;
 		goto error_return;
 	}
@@ -926,7 +927,7 @@ static struct ref_lock *lock_ref_sha1_basic(struct files_ref_store *refs,
 
 struct ref_to_prune {
 	struct ref_to_prune *next;
-	unsigned char sha1[20];
+	struct object_id oid;
 	char name[FLEX_ARRAY];
 };
 
@@ -994,7 +995,7 @@ static void prune_ref(struct files_ref_store *refs, struct ref_to_prune *r)
 
 	transaction = ref_store_transaction_begin(&refs->base, &err);
 	if (!transaction ||
-	    ref_transaction_delete(transaction, r->name, r->sha1,
+	    ref_transaction_delete(transaction, r->name, &r->oid,
 				   REF_ISPRUNING | REF_NODEREF, NULL, &err) ||
 	    ref_transaction_commit(transaction, &err)) {
 		ref_transaction_free(transaction);
@@ -1079,7 +1080,7 @@ static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
 		 * packed-refs transaction:
 		 */
 		if (ref_transaction_update(transaction, iter->refname,
-					   iter->oid->hash, NULL,
+					   iter->oid, NULL,
 					   REF_NODEREF, NULL, &err))
 			die("failure preparing to create packed reference %s: %s",
 			    iter->refname, err.buf);
@@ -1088,7 +1089,7 @@ static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
 		if ((flags & PACK_REFS_PRUNE)) {
 			struct ref_to_prune *n;
 			FLEX_ALLOC_STR(n, name, iter->refname);
-			hashcpy(n->sha1, iter->oid->hash);
+			oidcpy(&n->oid, iter->oid);
 			n->next = refs_to_prune;
 			refs_to_prune = n;
 		}
@@ -1251,7 +1252,7 @@ static int files_copy_or_rename_ref(struct ref_store *ref_store,
 
 	if (!refs_resolve_ref_unsafe(&refs->base, oldrefname,
 				     RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
-				orig_oid.hash, &flag)) {
+				&orig_oid, &flag)) {
 		ret = error("refname %s not found", oldrefname);
 		goto out;
 	}
@@ -1283,7 +1284,7 @@ static int files_copy_or_rename_ref(struct ref_store *ref_store,
 	}
 
 	if (!copy && refs_delete_ref(&refs->base, logmsg, oldrefname,
-			    orig_oid.hash, REF_NODEREF)) {
+			    &orig_oid, REF_NODEREF)) {
 		error("unable to delete old %s", oldrefname);
 		goto rollback;
 	}
@@ -1297,7 +1298,7 @@ static int files_copy_or_rename_ref(struct ref_store *ref_store,
 	 */
 	if (!copy && !refs_read_ref_full(&refs->base, newrefname,
 				RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
-				oid.hash, NULL) &&
+				&oid, NULL) &&
 	    refs_delete_ref(&refs->base, NULL, newrefname,
 			    NULL, REF_NODEREF)) {
 		if (errno == EISDIR) {
@@ -1323,8 +1324,8 @@ static int files_copy_or_rename_ref(struct ref_store *ref_store,
 
 	logmoved = log;
 
-	lock = lock_ref_sha1_basic(refs, newrefname, NULL, NULL, NULL,
-				   REF_NODEREF, NULL, &err);
+	lock = lock_ref_oid_basic(refs, newrefname, NULL, NULL, NULL,
+				  REF_NODEREF, NULL, &err);
 	if (!lock) {
 		if (copy)
 			error("unable to copy '%s' to '%s': %s", oldrefname, newrefname, err.buf);
@@ -1346,8 +1347,8 @@ static int files_copy_or_rename_ref(struct ref_store *ref_store,
 	goto out;
 
  rollback:
-	lock = lock_ref_sha1_basic(refs, oldrefname, NULL, NULL, NULL,
-				   REF_NODEREF, NULL, &err);
+	lock = lock_ref_oid_basic(refs, oldrefname, NULL, NULL, NULL,
+				  REF_NODEREF, NULL, &err);
 	if (!lock) {
 		error("unable to lock %s for rollback: %s", oldrefname, err.buf);
 		strbuf_release(&err);
@@ -1721,7 +1722,7 @@ static void update_symref_reflog(struct files_ref_store *refs,
 	struct object_id new_oid;
 	if (logmsg &&
 	    !refs_read_ref_full(&refs->base, target,
-				RESOLVE_REF_READING, new_oid.hash, NULL) &&
+				RESOLVE_REF_READING, &new_oid, NULL) &&
 	    files_log_ref_write(refs, refname, &lock->old_oid,
 				&new_oid, logmsg, 0, &err)) {
 		error("%s", err.buf);
@@ -1762,9 +1763,9 @@ static int files_create_symref(struct ref_store *ref_store,
 	struct ref_lock *lock;
 	int ret;
 
-	lock = lock_ref_sha1_basic(refs, refname, NULL,
-				   NULL, NULL, REF_NODEREF, NULL,
-				   &err);
+	lock = lock_ref_oid_basic(refs, refname, NULL,
+				  NULL, NULL, REF_NODEREF, NULL,
+				  &err);
 	if (!lock) {
 		error("%s", err.buf);
 		strbuf_release(&err);
@@ -2010,7 +2011,7 @@ static int files_reflog_iterator_advance(struct ref_iterator *ref_iterator)
 
 		if (refs_read_ref_full(iter->ref_store,
 				       diter->relative_path, 0,
-				       iter->oid.hash, &flags)) {
+				       &iter->oid, &flags)) {
 			error("bad ref for %s", diter->path.buf);
 			continue;
 		}
@@ -2148,7 +2149,7 @@ static int split_head_update(struct ref_update *update,
 	new_update = ref_transaction_add_update(
 			transaction, "HEAD",
 			update->flags | REF_LOG_ONLY | REF_NODEREF,
-			update->new_oid.hash, update->old_oid.hash,
+			&update->new_oid, &update->old_oid,
 			update->msg);
 
 	/*
@@ -2212,7 +2213,7 @@ static int split_symref_update(struct files_ref_store *refs,
 
 	new_update = ref_transaction_add_update(
 			transaction, referent, new_flags,
-			update->new_oid.hash, update->old_oid.hash,
+			&update->new_oid, &update->old_oid,
 			update->msg);
 
 	new_update->parent_update = update;
@@ -2347,7 +2348,7 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 			 */
 			if (refs_read_ref_full(&refs->base,
 					       referent.buf, 0,
-					       lock->old_oid.hash, NULL)) {
+					       &lock->old_oid, NULL)) {
 				if (update->flags & REF_HAVE_OLD) {
 					strbuf_addf(err, "cannot lock ref '%s': "
 						    "error reading reference",
@@ -2594,7 +2595,7 @@ static int files_transaction_prepare(struct ref_store *ref_store,
 			ref_transaction_add_update(
 					packed_transaction, update->refname,
 					update->flags & ~REF_HAVE_OLD,
-					update->new_oid.hash, update->old_oid.hash,
+					&update->new_oid, &update->old_oid,
 					NULL);
 		}
 	}
@@ -2847,7 +2848,7 @@ static int files_initial_transaction_commit(struct ref_store *ref_store,
 		 */
 		ref_transaction_add_update(packed_transaction, update->refname,
 					   update->flags & ~REF_HAVE_OLD,
-					   update->new_oid.hash, update->old_oid.hash,
+					   &update->new_oid, &update->old_oid,
 					   NULL);
 	}
 
@@ -2908,7 +2909,7 @@ static int expire_reflog_ent(struct object_id *ooid, struct object_id *noid,
 }
 
 static int files_reflog_expire(struct ref_store *ref_store,
-			       const char *refname, const unsigned char *sha1,
+			       const char *refname, const struct object_id *oid,
 			       unsigned int flags,
 			       reflog_expiry_prepare_fn prepare_fn,
 			       reflog_expiry_should_prune_fn should_prune_fn,
@@ -2925,7 +2926,6 @@ static int files_reflog_expire(struct ref_store *ref_store,
 	int status = 0;
 	int type;
 	struct strbuf err = STRBUF_INIT;
-	struct object_id oid;
 
 	memset(&cb, 0, sizeof(cb));
 	cb.flags = flags;
@@ -2937,9 +2937,9 @@ static int files_reflog_expire(struct ref_store *ref_store,
 	 * reference itself, plus we might need to update the
 	 * reference if --updateref was specified:
 	 */
-	lock = lock_ref_sha1_basic(refs, refname, sha1,
-				   NULL, NULL, REF_NODEREF,
-				   &type, &err);
+	lock = lock_ref_oid_basic(refs, refname, oid,
+				  NULL, NULL, REF_NODEREF,
+				  &type, &err);
 	if (!lock) {
 		error("cannot lock ref '%s': %s", refname, err.buf);
 		strbuf_release(&err);
@@ -2975,9 +2975,7 @@ static int files_reflog_expire(struct ref_store *ref_store,
 		}
 	}
 
-	hashcpy(oid.hash, sha1);
-
-	(*prepare_fn)(refname, &oid, cb.policy_cb);
+	(*prepare_fn)(refname, oid, cb.policy_cb);
 	refs_for_each_reflog_ent(ref_store, refname, expire_reflog_ent, &cb);
 	(*cleanup_fn)(cb.policy_cb);
 
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index 3279d42c5a..74f1dea0f4 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -716,7 +716,7 @@ static struct snapshot *get_snapshot(struct packed_ref_store *refs)
 }
 
 static int packed_read_raw_ref(struct ref_store *ref_store,
-			       const char *refname, unsigned char *sha1,
+			       const char *refname, struct object_id *oid,
 			       struct strbuf *referent, unsigned int *type)
 {
 	struct packed_ref_store *refs =
@@ -734,7 +734,7 @@ static int packed_read_raw_ref(struct ref_store *ref_store,
 		return -1;
 	}
 
-	if (get_sha1_hex(rec, sha1))
+	if (get_oid_hex(rec, oid))
 		die_invalid_line(refs->path, rec, snapshot->eof - rec);
 
 	*type = REF_ISPACKED;
@@ -880,7 +880,7 @@ static int packed_ref_iterator_peel(struct ref_iterator *ref_iterator,
 	} else if ((iter->base.flags & (REF_ISBROKEN | REF_ISSYMREF))) {
 		return -1;
 	} else {
-		return !!peel_object(iter->oid.hash, peeled->hash);
+		return !!peel_object(&iter->oid, peeled);
 	}
 }
 
@@ -1220,8 +1220,8 @@ static int write_with_updates(struct packed_ref_store *refs,
 			i++;
 		} else {
 			struct object_id peeled;
-			int peel_error = peel_object(update->new_oid.hash,
-						     peeled.hash);
+			int peel_error = peel_object(&update->new_oid,
+						     &peeled);
 
 			if (write_packed_entry(out, update->refname,
 					       update->new_oid.hash,
@@ -1519,7 +1519,7 @@ static int packed_delete_reflog(struct ref_store *ref_store,
 }
 
 static int packed_reflog_expire(struct ref_store *ref_store,
-				const char *refname, const unsigned char *sha1,
+				const char *refname, const struct object_id *oid,
 				unsigned int flags,
 				reflog_expiry_prepare_fn prepare_fn,
 				reflog_expiry_should_prune_fn should_prune_fn,
diff --git a/refs/ref-cache.c b/refs/ref-cache.c
index 4f850e1b5c..043eb83748 100644
--- a/refs/ref-cache.c
+++ b/refs/ref-cache.c
@@ -493,7 +493,7 @@ static int cache_ref_iterator_advance(struct ref_iterator *ref_iterator)
 static int cache_ref_iterator_peel(struct ref_iterator *ref_iterator,
 				   struct object_id *peeled)
 {
-	return peel_object(ref_iterator->oid->hash, peeled->hash);
+	return peel_object(ref_iterator->oid, peeled);
 }
 
 static int cache_ref_iterator_abort(struct ref_iterator *ref_iterator)
diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index 448de4bccb..b0f3e300c7 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h
@@ -120,11 +120,11 @@ enum peel_status {
 /*
  * Peel the named object; i.e., if the object is a tag, resolve the
  * tag recursively until a non-tag is found.  If successful, store the
- * result to sha1 and return PEEL_PEELED.  If the object is not a tag
+ * result to oid and return PEEL_PEELED.  If the object is not a tag
  * or is not valid, return PEEL_NON_TAG or PEEL_INVALID, respectively,
  * and leave sha1 unchanged.
  */
-enum peel_status peel_object(const unsigned char *name, unsigned char *sha1);
+enum peel_status peel_object(const struct object_id *name, struct object_id *oid);
 
 /*
  * Copy the reflog message msg to buf, which has been allocated sufficiently
@@ -181,7 +181,7 @@ struct ref_update {
 };
 
 int refs_read_raw_ref(struct ref_store *ref_store,
-		      const char *refname, unsigned char *sha1,
+		      const char *refname, struct object_id *oid,
 		      struct strbuf *referent, unsigned int *type);
 
 /*
@@ -202,8 +202,8 @@ int ref_update_reject_duplicates(struct string_list *refnames,
 struct ref_update *ref_transaction_add_update(
 		struct ref_transaction *transaction,
 		const char *refname, unsigned int flags,
-		const unsigned char *new_sha1,
-		const unsigned char *old_sha1,
+		const struct object_id *new_oid,
+		const struct object_id *old_oid,
 		const char *msg);
 
 /*
@@ -608,7 +608,7 @@ typedef int create_reflog_fn(struct ref_store *ref_store, const char *refname,
 			     int force_create, struct strbuf *err);
 typedef int delete_reflog_fn(struct ref_store *ref_store, const char *refname);
 typedef int reflog_expire_fn(struct ref_store *ref_store,
-			     const char *refname, const unsigned char *sha1,
+			     const char *refname, const struct object_id *oid,
 			     unsigned int flags,
 			     reflog_expiry_prepare_fn prepare_fn,
 			     reflog_expiry_should_prune_fn should_prune_fn,
@@ -619,13 +619,13 @@ typedef int reflog_expire_fn(struct ref_store *ref_store,
  * Read a reference from the specified reference store, non-recursively.
  * Set type to describe the reference, and:
  *
- * - If refname is the name of a normal reference, fill in sha1
+ * - If refname is the name of a normal reference, fill in oid
  *   (leaving referent unchanged).
  *
  * - If refname is the name of a symbolic reference, write the full
  *   name of the reference to which it refers (e.g.
  *   "refs/heads/master") to referent and set the REF_ISSYMREF bit in
- *   type (leaving sha1 unchanged). The caller is responsible for
+ *   type (leaving oid unchanged). The caller is responsible for
  *   validating that referent is a valid reference name.
  *
  * WARNING: refname might be used as part of a filename, so it is
@@ -637,7 +637,7 @@ typedef int reflog_expire_fn(struct ref_store *ref_store,
  *
  * Return 0 on success. If the ref doesn't exist, set errno to ENOENT
  * and return -1. If the ref exists but is neither a symbolic ref nor
- * a sha1, it is broken; set REF_ISBROKEN in type, set errno to
+ * an object ID, it is broken; set REF_ISBROKEN in type, set errno to
  * EINVAL, and return -1. If there is another error reading the ref,
  * set errno appropriately and return -1.
  *
@@ -654,7 +654,7 @@ typedef int reflog_expire_fn(struct ref_store *ref_store,
  *   refname will still be valid and unchanged.
  */
 typedef int read_raw_ref_fn(struct ref_store *ref_store,
-			    const char *refname, unsigned char *sha1,
+			    const char *refname, struct object_id *oid,
 			    struct strbuf *referent, unsigned int *type);
 
 struct ref_storage_be {
diff --git a/remote-testsvn.c b/remote-testsvn.c
index 0ff4a31262..bcebb4c789 100644
--- a/remote-testsvn.c
+++ b/remote-testsvn.c
@@ -174,7 +174,7 @@ static int cmd_import(const char *line)
 	struct child_process svndump_proc = CHILD_PROCESS_INIT;
 	const char *command = "svnrdump";
 
-	if (read_ref(private_ref, head_oid.hash))
+	if (read_ref(private_ref, &head_oid))
 		startrev = 0;
 	else {
 		note_msg = read_ref_note(&head_oid);
diff --git a/remote.c b/remote.c
index b220f0dfc6..685e776a65 100644
--- a/remote.c
+++ b/remote.c
@@ -1629,7 +1629,7 @@ static void set_merge(struct branch *ret)
 		    strcmp(ret->remote_name, "."))
 			continue;
 		if (dwim_ref(ret->merge_name[i], strlen(ret->merge_name[i]),
-			     oid.hash, &ref) == 1)
+			     &oid, &ref) == 1)
 			ret->merge[i]->dst = ref;
 		else
 			ret->merge[i]->dst = xstrdup(ret->merge_name[i]);
@@ -2002,13 +2002,13 @@ int stat_tracking_info(struct branch *branch, int *num_ours, int *num_theirs,
 		return -1;
 
 	/* Cannot stat if what we used to build on no longer exists */
-	if (read_ref(base, oid.hash))
+	if (read_ref(base, &oid))
 		return -1;
 	theirs = lookup_commit_reference(&oid);
 	if (!theirs)
 		return -1;
 
-	if (read_ref(branch->refname, oid.hash))
+	if (read_ref(branch->refname, &oid))
 		return -1;
 	ours = lookup_commit_reference(&oid);
 	if (!ours)
@@ -2327,7 +2327,7 @@ static int remote_tracking(struct remote *remote, const char *refname,
 	dst = apply_refspecs(remote->fetch, remote->fetch_refspec_nr, refname);
 	if (!dst)
 		return -1; /* no tracking ref for refname at remote */
-	if (read_ref(dst, oid->hash))
+	if (read_ref(dst, oid))
 		return -1; /* we know what the tracking ref is but we cannot read it */
 	return 0;
 }
diff --git a/sequencer.c b/sequencer.c
index 7c874be1c6..1eb2c4669d 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -393,7 +393,7 @@ static int fast_forward_to(const struct object_id *to, const struct object_id *f
 	transaction = ref_transaction_begin(&err);
 	if (!transaction ||
 	    ref_transaction_update(transaction, "HEAD",
-				   to->hash, unborn ? null_sha1 : from->hash,
+				   to, unborn ? &null_oid : from,
 				   0, sb.buf, &err) ||
 	    ref_transaction_commit(transaction, &err)) {
 		ref_transaction_free(transaction);
@@ -489,7 +489,7 @@ static int is_index_unchanged(void)
 	struct object_id head_oid;
 	struct commit *head_commit;
 
-	if (!resolve_ref_unsafe("HEAD", RESOLVE_REF_READING, head_oid.hash, NULL))
+	if (!resolve_ref_unsafe("HEAD", RESOLVE_REF_READING, &head_oid, NULL))
 		return error(_("could not resolve HEAD commit\n"));
 
 	head_commit = lookup_commit(&head_oid);
@@ -1115,11 +1115,11 @@ static int do_pick_commit(enum todo_command command, struct commit *commit,
 	 * write it at all.
 	 */
 	if (command == TODO_PICK && !opts->no_commit && (res == 0 || res == 1) &&
-	    update_ref(NULL, "CHERRY_PICK_HEAD", commit->object.oid.hash, NULL,
+	    update_ref(NULL, "CHERRY_PICK_HEAD", &commit->object.oid, NULL,
 		       REF_NODEREF, UPDATE_REFS_MSG_ON_ERR))
 		res = -1;
 	if (command == TODO_REVERT && ((opts->no_commit && res == 0) || res == 1) &&
-	    update_ref(NULL, "REVERT_HEAD", commit->object.oid.hash, NULL,
+	    update_ref(NULL, "REVERT_HEAD", &commit->object.oid, NULL,
 		       REF_NODEREF, UPDATE_REFS_MSG_ON_ERR))
 		res = -1;
 
@@ -1629,7 +1629,7 @@ static int rollback_single_pick(void)
 	if (!file_exists(git_path_cherry_pick_head()) &&
 	    !file_exists(git_path_revert_head()))
 		return error(_("no cherry-pick or revert in progress"));
-	if (read_ref_full("HEAD", 0, head_oid.hash, NULL))
+	if (read_ref_full("HEAD", 0, &head_oid, NULL))
 		return error(_("cannot resolve HEAD"));
 	if (is_null_oid(&head_oid))
 		return error(_("cannot abort from a branch yet to be born"));
@@ -2128,8 +2128,8 @@ cleanup_head_ref:
 			}
 			msg = reflog_message(opts, "finish", "%s onto %s",
 				head_ref.buf, buf.buf);
-			if (update_ref(msg, head_ref.buf, head.hash, orig.hash,
-					REF_NODEREF, UPDATE_REFS_MSG_ON_ERR)) {
+			if (update_ref(msg, head_ref.buf, &head, &orig,
+				       REF_NODEREF, UPDATE_REFS_MSG_ON_ERR)) {
 				res = error(_("could not update %s"),
 					head_ref.buf);
 				goto cleanup_head_ref;
diff --git a/sha1_file.c b/sha1_file.c
index 00f5b9e0b2..d7de8184ed 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -1661,7 +1661,7 @@ static void check_tag(const void *buf, size_t size)
 		die("corrupt tag");
 }
 
-static int index_mem(unsigned char *sha1, void *buf, size_t size,
+static int index_mem(struct object_id *oid, void *buf, size_t size,
 		     enum object_type type,
 		     const char *path, unsigned flags)
 {
@@ -1692,15 +1692,15 @@ static int index_mem(unsigned char *sha1, void *buf, size_t size,
 	}
 
 	if (write_object)
-		ret = write_sha1_file(buf, size, typename(type), sha1);
+		ret = write_sha1_file(buf, size, typename(type), oid->hash);
 	else
-		ret = hash_sha1_file(buf, size, typename(type), sha1);
+		ret = hash_sha1_file(buf, size, typename(type), oid->hash);
 	if (re_allocated)
 		free(buf);
 	return ret;
 }
 
-static int index_stream_convert_blob(unsigned char *sha1, int fd,
+static int index_stream_convert_blob(struct object_id *oid, int fd,
 				     const char *path, unsigned flags)
 {
 	int ret;
@@ -1715,22 +1715,22 @@ static int index_stream_convert_blob(unsigned char *sha1, int fd,
 
 	if (write_object)
 		ret = write_sha1_file(sbuf.buf, sbuf.len, typename(OBJ_BLOB),
-				      sha1);
+				      oid->hash);
 	else
 		ret = hash_sha1_file(sbuf.buf, sbuf.len, typename(OBJ_BLOB),
-				     sha1);
+				     oid->hash);
 	strbuf_release(&sbuf);
 	return ret;
 }
 
-static int index_pipe(unsigned char *sha1, int fd, enum object_type type,
+static int index_pipe(struct object_id *oid, int fd, enum object_type type,
 		      const char *path, unsigned flags)
 {
 	struct strbuf sbuf = STRBUF_INIT;
 	int ret;
 
 	if (strbuf_read(&sbuf, fd, 4096) >= 0)
-		ret = index_mem(sha1, sbuf.buf, sbuf.len, type,	path, flags);
+		ret = index_mem(oid, sbuf.buf, sbuf.len, type, path, flags);
 	else
 		ret = -1;
 	strbuf_release(&sbuf);
@@ -1739,14 +1739,14 @@ static int index_pipe(unsigned char *sha1, int fd, enum object_type type,
 
 #define SMALL_FILE_SIZE (32*1024)
 
-static int index_core(unsigned char *sha1, int fd, size_t size,
+static int index_core(struct object_id *oid, int fd, size_t size,
 		      enum object_type type, const char *path,
 		      unsigned flags)
 {
 	int ret;
 
 	if (!size) {
-		ret = index_mem(sha1, "", size, type, path, flags);
+		ret = index_mem(oid, "", size, type, path, flags);
 	} else if (size <= SMALL_FILE_SIZE) {
 		char *buf = xmalloc(size);
 		ssize_t read_result = read_in_full(fd, buf, size);
@@ -1757,11 +1757,11 @@ static int index_core(unsigned char *sha1, int fd, size_t size,
 			ret = error("short read while indexing %s",
 				    path ? path : "<unknown>");
 		else
-			ret = index_mem(sha1, buf, size, type, path, flags);
+			ret = index_mem(oid, buf, size, type, path, flags);
 		free(buf);
 	} else {
 		void *buf = xmmap(NULL, size, PROT_READ, MAP_PRIVATE, fd, 0);
-		ret = index_mem(sha1, buf, size, type, path, flags);
+		ret = index_mem(oid, buf, size, type, path, flags);
 		munmap(buf, size);
 	}
 	return ret;
@@ -1799,12 +1799,12 @@ int index_fd(struct object_id *oid, int fd, struct stat *st,
 	 * die() for large files.
 	 */
 	if (type == OBJ_BLOB && path && would_convert_to_git_filter_fd(path))
-		ret = index_stream_convert_blob(oid->hash, fd, path, flags);
+		ret = index_stream_convert_blob(oid, fd, path, flags);
 	else if (!S_ISREG(st->st_mode))
-		ret = index_pipe(oid->hash, fd, type, path, flags);
+		ret = index_pipe(oid, fd, type, path, flags);
 	else if (st->st_size <= big_file_threshold || type != OBJ_BLOB ||
 		 (path && would_convert_to_git(&the_index, path)))
-		ret = index_core(oid->hash, fd, xsize_t(st->st_size), type, path,
+		ret = index_core(oid, fd, xsize_t(st->st_size), type, path,
 				 flags);
 	else
 		ret = index_stream(oid, fd, xsize_t(st->st_size), type, path,
@@ -1838,7 +1838,7 @@ int index_path(struct object_id *oid, const char *path, struct stat *st, unsigne
 		strbuf_release(&sb);
 		break;
 	case S_IFDIR:
-		return resolve_gitlink_ref(path, "HEAD", oid->hash);
+		return resolve_gitlink_ref(path, "HEAD", oid);
 	default:
 		return error("%s: unsupported file type", path);
 	}
diff --git a/sha1_name.c b/sha1_name.c
index 2d6b48047b..f3b53f2c29 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -706,7 +706,7 @@ static int get_oid_basic(const char *str, int len, struct object_id *oid,
 
 	if (len == GIT_SHA1_HEXSZ && !get_oid_hex(str, oid)) {
 		if (warn_ambiguous_refs && warn_on_object_refname_ambiguity) {
-			refs_found = dwim_ref(str, len, tmp_oid.hash, &real_ref);
+			refs_found = dwim_ref(str, len, &tmp_oid, &real_ref);
 			if (refs_found > 0) {
 				warning(warn_msg, len, str);
 				if (advice_object_name_warning)
@@ -757,11 +757,11 @@ static int get_oid_basic(const char *str, int len, struct object_id *oid,
 
 	if (!len && reflog_len)
 		/* allow "@{...}" to mean the current branch reflog */
-		refs_found = dwim_ref("HEAD", 4, oid->hash, &real_ref);
+		refs_found = dwim_ref("HEAD", 4, oid, &real_ref);
 	else if (reflog_len)
-		refs_found = dwim_log(str, len, oid->hash, &real_ref);
+		refs_found = dwim_log(str, len, oid, &real_ref);
 	else
-		refs_found = dwim_ref(str, len, oid->hash, &real_ref);
+		refs_found = dwim_ref(str, len, oid, &real_ref);
 
 	if (!refs_found)
 		return -1;
@@ -800,7 +800,7 @@ static int get_oid_basic(const char *str, int len, struct object_id *oid,
 				return -1;
 			}
 		}
-		if (read_ref_at(real_ref, flags, at_time, nth, oid->hash, NULL,
+		if (read_ref_at(real_ref, flags, at_time, nth, oid, NULL,
 				&co_time, &co_tz, &co_cnt)) {
 			if (!len) {
 				if (starts_with(real_ref, "refs/heads/")) {
diff --git a/submodule.c b/submodule.c
index 63e7094e16..3b7be4cafe 100644
--- a/submodule.c
+++ b/submodule.c
@@ -1016,7 +1016,7 @@ int push_unpushed_submodules(struct oid_array *commits,
 		char *head;
 		struct object_id head_oid;
 
-		head = resolve_refdup("HEAD", 0, head_oid.hash, NULL);
+		head = resolve_refdup("HEAD", 0, &head_oid, NULL);
 		if (!head)
 			die(_("Failed to resolve HEAD as a valid ref."));
 
diff --git a/t/helper/test-ref-store.c b/t/helper/test-ref-store.c
index 6ec2670044..7120634b04 100644
--- a/t/helper/test-ref-store.c
+++ b/t/helper/test-ref-store.c
@@ -72,12 +72,12 @@ static int cmd_pack_refs(struct ref_store *refs, const char **argv)
 static int cmd_peel_ref(struct ref_store *refs, const char **argv)
 {
 	const char *refname = notnull(*argv++, "refname");
-	unsigned char sha1[20];
+	struct object_id oid;
 	int ret;
 
-	ret = refs_peel_ref(refs, refname, sha1);
+	ret = refs_peel_ref(refs, refname, &oid);
 	if (!ret)
-		puts(sha1_to_hex(sha1));
+		puts(oid_to_hex(&oid));
 	return ret;
 }
 
@@ -127,15 +127,15 @@ static int cmd_for_each_ref(struct ref_store *refs, const char **argv)
 
 static int cmd_resolve_ref(struct ref_store *refs, const char **argv)
 {
-	unsigned char sha1[20];
+	struct object_id oid;
 	const char *refname = notnull(*argv++, "refname");
 	int resolve_flags = arg_flags(*argv++, "resolve-flags");
 	int flags;
 	const char *ref;
 
 	ref = refs_resolve_ref_unsafe(refs, refname, resolve_flags,
-				      sha1, &flags);
-	printf("%s %s 0x%x\n", sha1_to_hex(sha1), ref ? ref : "(null)", flags);
+				      &oid, &flags);
+	printf("%s %s 0x%x\n", oid_to_hex(&oid), ref ? ref : "(null)", flags);
 	return ref ? 0 : 1;
 }
 
@@ -218,12 +218,12 @@ static int cmd_delete_ref(struct ref_store *refs, const char **argv)
 	const char *refname = notnull(*argv++, "refname");
 	const char *sha1_buf = notnull(*argv++, "old-sha1");
 	unsigned int flags = arg_flags(*argv++, "flags");
-	unsigned char old_sha1[20];
+	struct object_id old_oid;
 
-	if (get_sha1_hex(sha1_buf, old_sha1))
+	if (get_oid_hex(sha1_buf, &old_oid))
 		die("not sha-1");
 
-	return refs_delete_ref(refs, msg, refname, old_sha1, flags);
+	return refs_delete_ref(refs, msg, refname, &old_oid, flags);
 }
 
 static int cmd_update_ref(struct ref_store *refs, const char **argv)
@@ -233,15 +233,15 @@ static int cmd_update_ref(struct ref_store *refs, const char **argv)
 	const char *new_sha1_buf = notnull(*argv++, "old-sha1");
 	const char *old_sha1_buf = notnull(*argv++, "old-sha1");
 	unsigned int flags = arg_flags(*argv++, "flags");
-	unsigned char old_sha1[20];
-	unsigned char new_sha1[20];
+	struct object_id old_oid;
+	struct object_id new_oid;
 
-	if (get_sha1_hex(old_sha1_buf, old_sha1) ||
-	    get_sha1_hex(new_sha1_buf, new_sha1))
+	if (get_oid_hex(old_sha1_buf, &old_oid) ||
+	    get_oid_hex(new_sha1_buf, &new_oid))
 		die("not sha-1");
 
 	return refs_update_ref(refs, msg, refname,
-			       new_sha1, old_sha1,
+			       &new_oid, &old_oid,
 			       flags, UPDATE_REFS_DIE_ON_ERR);
 }
 
diff --git a/transport-helper.c b/transport-helper.c
index c948d5215c..bf05a2dcf1 100644
--- a/transport-helper.c
+++ b/transport-helper.c
@@ -535,7 +535,7 @@ static int fetch_with_import(struct transport *transport,
 		else
 			private = xstrdup(name);
 		if (private) {
-			if (read_ref(private, posn->old_oid.hash) < 0)
+			if (read_ref(private, &posn->old_oid) < 0)
 				die("Could not read ref %s", private);
 			free(private);
 		}
@@ -795,7 +795,8 @@ static int push_update_refs_status(struct helper_data *data,
 		private = apply_refspecs(data->refspecs, data->refspec_nr, ref->name);
 		if (!private)
 			continue;
-		update_ref("update by helper", private, ref->new_oid.hash, NULL, 0, 0);
+		update_ref("update by helper", private, &ref->new_oid, NULL,
+			   0, 0);
 		free(private);
 	}
 	strbuf_release(&buf);
@@ -941,10 +942,9 @@ static int push_refs_with_export(struct transport *transport,
 					int flag;
 
 					/* Follow symbolic refs (mainly for HEAD). */
-					name = resolve_ref_unsafe(
-						 ref->peer_ref->name,
-						 RESOLVE_REF_READING,
-						 oid.hash, &flag);
+					name = resolve_ref_unsafe(ref->peer_ref->name,
+								  RESOLVE_REF_READING,
+								  &oid, &flag);
 					if (!name || !(flag & REF_ISSYMREF))
 						name = ref->peer_ref->name;
 
@@ -1066,8 +1066,7 @@ static struct ref *get_refs_list(struct transport *transport, int for_push)
 		if (eon) {
 			if (has_attribute(eon + 1, "unchanged")) {
 				(*tail)->status |= REF_STATUS_UPTODATE;
-				if (read_ref((*tail)->name,
-					     (*tail)->old_oid.hash) < 0)
+				if (read_ref((*tail)->name, &(*tail)->old_oid) < 0)
 					die(_("Could not read ref %s"),
 					    (*tail)->name);
 			}
diff --git a/transport.c b/transport.c
index f1e2f61991..7231d1b1b0 100644
--- a/transport.c
+++ b/transport.c
@@ -305,8 +305,8 @@ void transport_update_tracking_ref(struct remote *remote, struct ref *ref, int v
 		if (ref->deletion) {
 			delete_ref(NULL, rs.dst, NULL, 0);
 		} else
-			update_ref("update by push", rs.dst,
-					ref->new_oid.hash, NULL, 0, 0);
+			update_ref("update by push", rs.dst, &ref->new_oid,
+				   NULL, 0, 0);
 		free(rs.dst);
 	}
 }
diff --git a/unpack-trees.c b/unpack-trees.c
index 71b70ccb12..25740cb593 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -1541,15 +1541,15 @@ static int verify_clean_subdirectory(const struct cache_entry *ce,
 	int cnt = 0;
 
 	if (S_ISGITLINK(ce->ce_mode)) {
-		unsigned char sha1[20];
-		int sub_head = resolve_gitlink_ref(ce->name, "HEAD", sha1);
+		struct object_id oid;
+		int sub_head = resolve_gitlink_ref(ce->name, "HEAD", &oid);
 		/*
 		 * If we are not going to update the submodule, then
 		 * we don't care.
 		 */
-		if (!sub_head && !hashcmp(sha1, ce->oid.hash))
+		if (!sub_head && !oidcmp(&oid, &ce->oid))
 			return 0;
-		return verify_clean_submodule(sub_head ? NULL : sha1_to_hex(sha1),
+		return verify_clean_submodule(sub_head ? NULL : oid_to_hex(&oid),
 					      ce, error_type, o);
 	}
 
diff --git a/upload-pack.c b/upload-pack.c
index e25f725c0f..6d5f3c0d39 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -787,7 +787,7 @@ static void receive_needs(void)
 		if (skip_prefix(line, "deepen-not ", &arg)) {
 			char *ref = NULL;
 			struct object_id oid;
-			if (expand_ref(arg, strlen(arg), oid.hash, &ref) != 1)
+			if (expand_ref(arg, strlen(arg), &oid, &ref) != 1)
 				die("git upload-pack: ambiguous deepen-not: %s", line);
 			string_list_append(&deepen_not, ref);
 			free(ref);
@@ -955,7 +955,7 @@ static int send_ref(const char *refname, const struct object_id *oid,
 		packet_write_fmt(1, "%s %s\n", oid_to_hex(oid), refname_nons);
 	}
 	capabilities = NULL;
-	if (!peel_ref(refname, peeled.hash))
+	if (!peel_ref(refname, &peeled))
 		packet_write_fmt(1, "%s %s^{}\n", oid_to_hex(&peeled), refname_nons);
 	return 0;
 }
diff --git a/walker.c b/walker.c
index 274f1a4935..5d4d3733f7 100644
--- a/walker.c
+++ b/walker.c
@@ -7,7 +7,7 @@
 #include "blob.h"
 #include "refs.h"
 
-static unsigned char current_commit_sha1[20];
+static struct object_id current_commit_oid;
 
 void walker_say(struct walker *walker, const char *fmt, ...)
 {
@@ -24,9 +24,9 @@ static void report_missing(const struct object *obj)
 	fprintf(stderr, "Cannot obtain needed %s %s\n",
 		obj->type ? typename(obj->type): "object",
 		oid_to_hex(&obj->oid));
-	if (!is_null_sha1(current_commit_sha1))
+	if (!is_null_oid(&current_commit_oid))
 		fprintf(stderr, "while processing commit %s.\n",
-			sha1_to_hex(current_commit_sha1));
+			oid_to_hex(&current_commit_oid));
 }
 
 static int process(struct walker *walker, struct object *obj);
@@ -82,7 +82,7 @@ static int process_commit(struct walker *walker, struct commit *commit)
 	if (commit->object.flags & COMPLETE)
 		return 0;
 
-	hashcpy(current_commit_sha1, commit->object.oid.hash);
+	oidcpy(&current_commit_oid, &commit->object.oid);
 
 	walker_say(walker, "walk %s\n", oid_to_hex(&commit->object.oid));
 
@@ -187,14 +187,14 @@ static int loop(struct walker *walker)
 	return 0;
 }
 
-static int interpret_target(struct walker *walker, char *target, unsigned char *sha1)
+static int interpret_target(struct walker *walker, char *target, struct object_id *oid)
 {
-	if (!get_sha1_hex(target, sha1))
+	if (!get_oid_hex(target, oid))
 		return 0;
 	if (!check_refname_format(target, 0)) {
 		struct ref *ref = alloc_ref(target);
 		if (!walker->fetch_ref(walker, ref)) {
-			hashcpy(sha1, ref->old_oid.hash);
+			oidcpy(oid, &ref->old_oid);
 			free(ref);
 			return 0;
 		}
@@ -259,7 +259,7 @@ int walker_fetch(struct walker *walker, int targets, char **target,
 	struct strbuf refname = STRBUF_INIT;
 	struct strbuf err = STRBUF_INIT;
 	struct ref_transaction *transaction = NULL;
-	unsigned char *sha1 = xmalloc(targets * 20);
+	struct object_id *oids = xmalloc(targets * sizeof(struct object_id));
 	char *msg = NULL;
 	int i, ret = -1;
 
@@ -279,11 +279,11 @@ int walker_fetch(struct walker *walker, int targets, char **target,
 	}
 
 	for (i = 0; i < targets; i++) {
-		if (interpret_target(walker, target[i], &sha1[20 * i])) {
+		if (interpret_target(walker, target[i], oids + i)) {
 			error("Could not interpret response from server '%s' as something to pull", target[i]);
 			goto done;
 		}
-		if (process(walker, lookup_unknown_object(&sha1[20 * i])))
+		if (process(walker, lookup_unknown_object(oids[i].hash)))
 			goto done;
 	}
 
@@ -304,7 +304,7 @@ int walker_fetch(struct walker *walker, int targets, char **target,
 		strbuf_reset(&refname);
 		strbuf_addf(&refname, "refs/%s", write_ref[i]);
 		if (ref_transaction_update(transaction, refname.buf,
-					   &sha1[20 * i], NULL, 0,
+					   oids + i, NULL, 0,
 					   msg ? msg : "fetch (unknown)",
 					   &err)) {
 			error("%s", err.buf);
@@ -321,7 +321,7 @@ int walker_fetch(struct walker *walker, int targets, char **target,
 done:
 	ref_transaction_free(transaction);
 	free(msg);
-	free(sha1);
+	free(oids);
 	strbuf_release(&err);
 	strbuf_release(&refname);
 	return ret;
diff --git a/worktree.c b/worktree.c
index f8c40f2f5f..f5da7d286d 100644
--- a/worktree.c
+++ b/worktree.c
@@ -31,7 +31,7 @@ static void add_head_info(struct worktree *wt)
 	target = refs_resolve_ref_unsafe(get_worktree_ref_store(wt),
 					 "HEAD",
 					 0,
-					 wt->head_sha1, &flags);
+					 &wt->head_oid, &flags);
 	if (!target)
 		return;
 
diff --git a/worktree.h b/worktree.h
index 9276c81ae7..c28a880e18 100644
--- a/worktree.h
+++ b/worktree.h
@@ -8,7 +8,7 @@ struct worktree {
 	char *id;
 	char *head_ref;		/* NULL if HEAD is broken or detached */
 	char *lock_reason;	/* internal use */
-	unsigned char head_sha1[20];
+	struct object_id head_oid;
 	int is_detached;
 	int is_bare;
 	int is_current;
diff --git a/wt-status.c b/wt-status.c
index 93ac645225..bedef256ce 100644
--- a/wt-status.c
+++ b/wt-status.c
@@ -1449,7 +1449,7 @@ static void wt_status_get_detached_from(struct wt_status_state *state)
 		return;
 	}
 
-	if (dwim_ref(cb.buf.buf, cb.buf.len, oid.hash, &ref) == 1 &&
+	if (dwim_ref(cb.buf.buf, cb.buf.len, &oid, &ref) == 1 &&
 	    /* sha1 is a commit? match without further lookup */
 	    (!oidcmp(&cb.noid, &oid) ||
 	     /* perhaps sha1 is a tag, try to dereference to a commit */