diff --git a/Documentation/git-mergetool.txt b/Documentation/git-mergetool.txt
index c4c1a9bde4..3622d66488 100644
--- a/Documentation/git-mergetool.txt
+++ b/Documentation/git-mergetool.txt
@@ -79,10 +79,12 @@ success of the resolution after the custom tool has exited.
 	Prompt before each invocation of the merge resolution program
 	to give the user a chance to skip the path.
 
-DIFF ORDER FILES
-----------------
-`git mergetool` honors the `diff.orderFile` configuration variable
-used by `git diff`.  See linkgit:git-config[1] for more details.
+-O<orderfile>::
+	Process files in the order specified in the
+	<orderfile>, which has one shell glob pattern per line.
+	This overrides the `diff.orderFile` configuration variable
+	(see linkgit:git-config[1]).  To cancel `diff.orderFile`,
+	use `-O/dev/null`.
 
 TEMPORARY FILES
 ---------------
diff --git a/git-mergetool.sh b/git-mergetool.sh
index 65696d8382..e52b4e4f24 100755
--- a/git-mergetool.sh
+++ b/git-mergetool.sh
@@ -9,7 +9,7 @@
 # at the discretion of Junio C Hamano.
 #
 
-USAGE='[--tool=tool] [--tool-help] [-y|--no-prompt|--prompt] [file to merge] ...'
+USAGE='[--tool=tool] [--tool-help] [-y|--no-prompt|--prompt] [-O<orderfile>] [file to merge] ...'
 SUBDIRECTORY_OK=Yes
 NONGIT_OK=Yes
 OPTIONS_SPEC=
@@ -390,6 +390,7 @@ print_noop_and_exit () {
 main () {
 	prompt=$(git config --bool mergetool.prompt)
 	guessed_merge_tool=false
+	orderfile=
 
 	while test $# != 0
 	do
@@ -419,6 +420,9 @@ main () {
 		--prompt)
 			prompt=true
 			;;
+		-O*)
+			orderfile="$1"
+			;;
 		--)
 			shift
 			break
@@ -460,7 +464,8 @@ main () {
 	fi
 
 	files=$(git -c core.quotePath=false \
-		diff --name-only --diff-filter=U -- "$@")
+		diff --name-only --diff-filter=U \
+		${orderfile:+"$orderfile"} -- "$@")
 
 	cd_to_toplevel
 
diff --git a/t/t7610-mergetool.sh b/t/t7610-mergetool.sh
index 38c1e4de71..6d9f21511f 100755
--- a/t/t7610-mergetool.sh
+++ b/t/t7610-mergetool.sh
@@ -638,5 +638,32 @@ test_expect_success 'diff.orderFile configuration is honored' '
 	test_cmp expect actual &&
 	git reset --hard >/dev/null
 '
+test_expect_success 'mergetool -Oorder-file is honored' '
+	test_config diff.orderFile order-file &&
+	test_config mergetool.myecho.cmd "echo \"\$LOCAL\"" &&
+	test_config mergetool.myecho.trustExitCode true &&
+	test_must_fail git merge order-file-side1 &&
+	cat >expect <<-\EOF &&
+		Merging:
+		a
+		b
+	EOF
+	git mergetool -O/dev/null --no-prompt --tool myecho >output &&
+	git grep --no-index -h -A2 Merging: output >actual &&
+	test_cmp expect actual &&
+	git reset --hard >/dev/null 2>&1 &&
+
+	git config --unset diff.orderFile &&
+	test_must_fail git merge order-file-side1 &&
+	cat >expect <<-\EOF &&
+		Merging:
+		b
+		a
+	EOF
+	git mergetool -Oorder-file --no-prompt --tool myecho >output &&
+	git grep --no-index -h -A2 Merging: output >actual &&
+	test_cmp expect actual &&
+	git reset --hard >/dev/null 2>&1
+'
 
 test_done