diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 42cb5ed..3d284b7 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -20,6 +20,8 @@ jobs:
             cache: pip
         - run: pip install pytest
         - run: pytest . --junitxml=junit/test_py${{ matrix.python-version }}_on_${{ matrix.os }}.xml
+        - run: python -O once_test.py
+        - run: python -OO once_test.py
         - name: Upload pytest test results
           uses: actions/upload-artifact@v3
           if: success() || failure()
diff --git a/once_test.py b/once_test.py
index 1fa83d5..74e096f 100644
--- a/once_test.py
+++ b/once_test.py
@@ -526,16 +526,22 @@ def closure():
         self.assertIsNone(ephemeral_ref())
 
     def test_function_signature_preserved(self):
-        @once.once
         def type_annotated_fn(arg: float) -> int:
             """Very descriptive docstring."""
             del arg
             return 1
 
-        sig = inspect.signature(type_annotated_fn)
-        self.assertIs(sig.parameters["arg"].annotation, float)
-        self.assertIs(sig.return_annotation, int)
-        self.assertEqual(type_annotated_fn.__doc__, "Very descriptive docstring.")
+        decorated_function = once.once(type_annotated_fn)
+        original_sig = inspect.signature(type_annotated_fn)
+        decorated_sig = inspect.signature(decorated_function)
+        self.assertIs(original_sig.parameters["arg"].annotation, float)
+        self.assertIs(decorated_sig.parameters["arg"].annotation, float)
+        self.assertIs(original_sig.return_annotation, int)
+        self.assertIs(decorated_sig.return_annotation, int)
+        self.assertEqual(inspect.getdoc(type_annotated_fn), inspect.getdoc(decorated_function))
+        if sys.flags.optimize >= 2:
+            self.skipTest("docstrings get stripped with -OO")
+        self.assertEqual(inspect.getdoc(type_annotated_fn), "Very descriptive docstring.")
 
     def test_once_per_class(self):
         class _CallOnceClass(Counter):