diff --git a/vllm/entrypoints/openai/cli_args.py b/vllm/entrypoints/openai/cli_args.py
index 6b2d0b625f043..c62ce5692f4a7 100644
--- a/vllm/entrypoints/openai/cli_args.py
+++ b/vllm/entrypoints/openai/cli_args.py
@@ -135,10 +135,11 @@ def make_arg_parser(parser: FlexibleArgumentParser) -> FlexibleArgumentParser:
                         "for the specified model")
     parser.add_argument(
             "--orca-format",
-            type=nullable_str,
+            type=str,
             default="",
+            choices=["BIN", "TEXT", "JSON"],
             help='Enable ORCA metrics reporting in response header'
-                'select one or many of valid formats from [BIN, TEXT, JSON]',
+                'select one of valid formats from [BIN, TEXT, JSON]',
         )
     parser.add_argument(
         '--chat-template-content-format',