Based on my knowledge of the pytest codebase, I can provide the patch. The `--collect-only` option is defined in `src/_pytest/main.py` in the pytest repository. The fix involves adding `--co` as an alias to the existing `--collect-only` option.

--- a/src/_pytest/main.py
+++ b/src/_pytest/main.py
@@ -109,6 +109,7 @@ def pytest_addoption(parser: Parser) -> None:
     group.addoption(
         "--collect-only",
         "--collectonly",
+        "--co",
         action="store_true",
         help="only collect tests, don't execute them.",
     )