diff --git a/litellm/llms/oci/chat/transformation.py b/litellm/llms/oci/chat/transformation.py
index 72d01c34ded..627d0ae8072 100644
--- a/litellm/llms/oci/chat/transformation.py
+++ b/litellm/llms/oci/chat/transformation.py
@@ -88,15 +88,20 @@ STREAMING_TIMEOUT = 60 * 5
 def _model_uses_max_completion_tokens(model: str) -> bool:
     """Return True for OCI-hosted models that require ``maxCompletionTokens``.
 
-    Reasoning models on OCI (e.g. the OpenAI GPT-5 family) reject ``maxTokens``
-    with HTTP 400 and require ``maxCompletionTokens`` per OpenAI's reasoning-API
-    convention. Driven by ``supports_reasoning`` in
-    ``model_prices_and_context_window.json`` so new model families are picked
-    up via a catalog update rather than a code change.
+    OpenAI commercial models proxied through OCI (``openai.*``) reject
+    ``maxTokens`` with HTTP 400 on the reasoning families (gpt-5.x, o-series)
+    and accept ``maxCompletionTokens`` everywhere, so route the whole vendor
+    prefix to it rather than chasing each new release in
+    ``model_prices_and_context_window.json``. The ``openai.gpt-oss-*`` open
+    weights are served by OCI's own stack and keep ``maxTokens``. Any other
+    vendor falls back to the catalog's ``supports_reasoning`` flag.
     """
     if not model:
         return False
     name = model[4:] if model.lower().startswith("oci/") else model
+    lowered = name.lower()
+    if lowered.startswith("openai."):
+        return not lowered.startswith("openai.gpt-oss")
     return supports_reasoning(model=name, custom_llm_provider="oci")
 
 
diff --git a/tests/test_litellm/llms/oci/chat/test_oci_generic_chat.py b/tests/test_litellm/llms/oci/chat/test_oci_generic_chat.py
index 7583e3bc183..b2c93d0458f 100644
--- a/tests/test_litellm/llms/oci/chat/test_oci_generic_chat.py
+++ b/tests/test_litellm/llms/oci/chat/test_oci_generic_chat.py
@@ -382,6 +382,41 @@ class TestGpt5MaxCompletionTokens:
         assert _model_uses_max_completion_tokens("cohere.command-latest") is False
         assert _model_uses_max_completion_tokens("") is False
 
+    def test_helper_covers_openai_models_absent_from_catalog(self):
+        """OCI keeps adding OpenAI models (gpt-4.1, gpt-5.1..5.5, o-series)
+        faster than the litellm catalog tracks them. The vendor-prefix rule
+        must route them to maxCompletionTokens even with no catalog entry,
+        since OpenAI accepts max_completion_tokens on every chat model while
+        the reasoning families hard-reject max_tokens."""
+        import litellm
+        from litellm.llms.oci.chat.transformation import (
+            _model_uses_max_completion_tokens,
+        )
+
+        for name in (
+            "openai.gpt-5.2",
+            "openai.gpt-4.1",
+            "openai.o3",
+            "oci/openai.gpt-5.1-codex",
+        ):
+            assert f"oci/{name.removeprefix('oci/')}" not in litellm.model_cost
+            assert _model_uses_max_completion_tokens(name) is True
+
+        assert _model_uses_max_completion_tokens("openai.gpt-oss-20b") is False
+
+    def test_default_injection_uses_max_completion_tokens_for_uncataloged_gpt(self):
+        """Regression: with the injected default maxTokens, a GPT model absent
+        from the catalog got "maxTokens" on every request and OCI returned 400
+        ("Use 'max_completion_tokens' instead") even when the caller never set
+        max_tokens."""
+        from litellm.constants import DEFAULT_OCI_CHAT_MAX_TOKENS
+        from litellm.llms.oci.chat.transformation import OCIChatConfig, OCIVendors
+
+        cfg = OCIChatConfig()
+        out = cfg._get_optional_params(OCIVendors.GENERIC, {}, model="openai.gpt-5.2")
+        assert out.get("maxCompletionTokens") == DEFAULT_OCI_CHAT_MAX_TOKENS
+        assert "maxTokens" not in out
+
     def test_gpt5_routes_max_tokens_to_max_completion_tokens(
         self, _register_oci_gpt5_in_catalog
     ):