diff --git a/litellm/llms/openai/chat/o_series_transformation.py b/litellm/llms/openai/chat/o_series_transformation.py
index c7552d47985..b3f19357a33 100644
--- a/litellm/llms/openai/chat/o_series_transformation.py
+++ b/litellm/llms/openai/chat/o_series_transformation.py
@@ -19,12 +19,17 @@ from litellm import verbose_logger
 from litellm.litellm_core_utils.get_llm_provider_logic import get_llm_provider
 from litellm.types.llms.openai import AllMessageValues, ChatCompletionUserMessage
 from litellm.utils import (
+    is_explicitly_disabled_factory,
     supports_function_calling,
     supports_parallel_function_calling,
     supports_response_schema,
     supports_system_messages,
 )
 
+from .gpt_5_transformation import (
+    get_effort_level,  # pyright: ignore[reportUnknownVariableType]  # its signature takes the bare reasoning_effort dict
+    unsupported_reasoning_effort_message,
+)
 from .gpt_transformation import OpenAIGPTConfig
 
 
@@ -95,6 +100,19 @@ class OpenAIOSeriesConfig(OpenAIGPTConfig):
         model: str,
         drop_params: bool,
     ):
+        requested_effort: Final = get_effort_level(
+            non_default_params.get("reasoning_effort") or optional_params.get("reasoning_effort")  # pyright: ignore[reportUnknownMemberType]  # the request params arrive as a bare dict
+        )
+        if requested_effort is not None and is_explicitly_disabled_factory(
+            model=model, custom_llm_provider=None, key=f"supports_{requested_effort}_reasoning_effort"
+        ):
+            if not (litellm.drop_params or drop_params):
+                raise litellm.utils.UnsupportedParamsError(
+                    message=unsupported_reasoning_effort_message("reasoning_effort", requested_effort, model),
+                    status_code=400,
+                )
+            non_default_params.pop("reasoning_effort", None)  # pyright: ignore[reportUnknownMemberType]  # the request params arrive as a bare dict
+            optional_params.pop("reasoning_effort", None)  # pyright: ignore[reportUnknownMemberType]  # the request params arrive as a bare dict
         if "max_tokens" in non_default_params:
             optional_params["max_completion_tokens"] = non_default_params.pop("max_tokens")
         if "temperature" in non_default_params:
diff --git a/litellm/model_prices_and_context_window_backup.json b/litellm/model_prices_and_context_window_backup.json
index 891e247747e..9b3ab7fb3d6 100644
--- a/litellm/model_prices_and_context_window_backup.json
+++ b/litellm/model_prices_and_context_window_backup.json
@@ -44582,6 +44582,8 @@
         "supports_pdf_input": true,
         "supports_prompt_caching": true,
         "supports_reasoning": true,
+        "supports_minimal_reasoning_effort": false,
+        "supports_none_reasoning_effort": false,
         "supports_response_schema": true,
         "supports_tool_choice": true,
         "supports_vision": true,
@@ -44693,6 +44695,8 @@
         "supports_pdf_input": true,
         "supports_prompt_caching": true,
         "supports_reasoning": true,
+        "supports_minimal_reasoning_effort": false,
+        "supports_none_reasoning_effort": false,
         "supports_response_schema": true,
         "supports_tool_choice": true,
         "supports_vision": true,
diff --git a/model_prices_and_context_window.json b/model_prices_and_context_window.json
index 891e247747e..9b3ab7fb3d6 100644
--- a/model_prices_and_context_window.json
+++ b/model_prices_and_context_window.json
@@ -44582,6 +44582,8 @@
         "supports_pdf_input": true,
         "supports_prompt_caching": true,
         "supports_reasoning": true,
+        "supports_minimal_reasoning_effort": false,
+        "supports_none_reasoning_effort": false,
         "supports_response_schema": true,
         "supports_tool_choice": true,
         "supports_vision": true,
@@ -44693,6 +44695,8 @@
         "supports_pdf_input": true,
         "supports_prompt_caching": true,
         "supports_reasoning": true,
+        "supports_minimal_reasoning_effort": false,
+        "supports_none_reasoning_effort": false,
         "supports_response_schema": true,
         "supports_tool_choice": true,
         "supports_vision": true,
diff --git a/tests/unit/llms/openai/test_o_series_transformation.py b/tests/unit/llms/openai/test_o_series_transformation.py
index c62523cf756..4b908b551e4 100644
--- a/tests/unit/llms/openai/test_o_series_transformation.py
+++ b/tests/unit/llms/openai/test_o_series_transformation.py
@@ -1,3 +1,5 @@
+from collections.abc import Mapping
+from typing import Final
 from unittest.mock import patch
 
 import pytest
@@ -5,6 +7,7 @@ import pytest
 import litellm
 from litellm import ModelResponse
 from litellm.llms.openai.chat.o_series_transformation import OpenAIOSeriesConfig
+from litellm.utils import get_optional_params
 import os
 
 
@@ -180,3 +183,105 @@ def test_litellm_responses():
     print("response: ", response)
 
     assert isinstance(response.usage.completion_tokens_details, CompletionTokensDetails)
+
+
+def _o3_row_with(flags: Mapping[str, bool]) -> dict[str, object]:
+    return {
+        **{
+            field: value
+            for field, value in litellm.model_cost["o3"].items()
+            if not (field.startswith("supports_") and field.endswith("_reasoning_effort"))
+        },
+        **flags,
+    }
+
+
+@pytest.mark.parametrize(
+    ("flags", "level", "forwarded"),
+    [
+        ({}, "minimal", True),
+        ({"supports_minimal_reasoning_effort": False}, "minimal", False),
+        ({"supports_minimal_reasoning_effort": True}, "minimal", True),
+        ({"supports_none_reasoning_effort": False}, "none", False),
+        ({"supports_minimal_reasoning_effort": False}, "low", True),
+        ({"supports_xhigh_reasoning_effort": False}, "xhigh", False),
+        ({}, "xhigh", True),
+    ],
+)
+def test_the_o_series_row_flags_alone_decide_whether_a_chat_level_is_forwarded(
+    local_model_cost_map: None,
+    monkeypatch: pytest.MonkeyPatch,
+    flags: dict[str, bool],
+    level: str,
+    forwarded: bool,
+):
+    monkeypatch.setitem(litellm.model_cost, "o3", _o3_row_with(flags))
+    config: Final = OpenAIOSeriesConfig()
+    dropped: Final = config.map_openai_params(
+        non_default_params={"reasoning_effort": level, "max_tokens": 10},
+        optional_params={},
+        model="o3",
+        drop_params=True,
+    )
+    assert dropped.get("reasoning_effort") == (level if forwarded else None)
+    assert dropped["max_completion_tokens"] == 10
+    if forwarded:
+        assert (
+            config.map_openai_params(
+                non_default_params={"reasoning_effort": level},
+                optional_params={},
+                model="o3",
+                drop_params=False,
+            )["reasoning_effort"]
+            == level
+        )
+    else:
+        with pytest.raises(litellm.utils.UnsupportedParamsError, match=f"reasoning_effort={level} is not supported"):
+            config.map_openai_params(
+                non_default_params={"reasoning_effort": level},
+                optional_params={},
+                model="o3",
+                drop_params=False,
+            )
+
+
+@pytest.mark.parametrize("model", ["o3", "o3-2025-04-16"])
+def test_o3_rows_turn_minimal_and_none_off(local_model_cost_map: None, model: str):
+    """Pinned so the row-flag test above cannot go vacuous: OpenAI's chat completions endpoint answered
+    400 for reasoning_effort minimal and none on o3 and 200 for low, medium, high and xhigh, both through
+    a local proxy and straight from api.openai.com on 2026-11-07. When OpenAI adds a level, flip the rows
+    and this assertion together."""
+    row: Final = litellm.model_cost[model]
+    assert row["supports_minimal_reasoning_effort"] is False
+    assert row["supports_none_reasoning_effort"] is False
+
+
+@pytest.mark.parametrize(
+    ("non_default_params", "optional_params"),
+    [
+        ({"reasoning_effort": {"effort": "minimal", "summary": "auto"}}, {}),
+        ({}, {"reasoning_effort": "minimal"}),
+    ],
+)
+def test_o3_judges_every_form_of_a_requested_level(
+    local_model_cost_map: None, non_default_params: dict[str, object], optional_params: dict[str, object]
+):
+    assert "reasoning_effort" not in OpenAIOSeriesConfig().map_openai_params(
+        non_default_params=non_default_params,
+        optional_params=optional_params,
+        model="o3",
+        drop_params=True,
+    )
+
+
+@pytest.mark.parametrize("custom_llm_provider", ["openai", "azure"])
+def test_o3_chat_request_drops_minimal_under_drop_params(local_model_cost_map: None, custom_llm_provider: str):
+    assert "reasoning_effort" not in get_optional_params(
+        model="o3", custom_llm_provider=custom_llm_provider, reasoning_effort="minimal", drop_params=True
+    )
+
+
+@pytest.mark.parametrize("custom_llm_provider", ["openai", "azure"])
+def test_o3_chat_request_refuses_minimal_without_drop_params(local_model_cost_map: None, custom_llm_provider: str):
+    with pytest.raises(litellm.utils.UnsupportedParamsError, match="reasoning_effort=minimal is not supported"):
+        get_optional_params(model="o3", custom_llm_provider=custom_llm_provider, reasoning_effort="minimal")
diff --git a/tests/unit/router_utils/test_reasoning_effort_capability.py b/tests/unit/router_utils/test_reasoning_effort_capability.py
index 2226b1f39df..8e561855207 100644
--- a/tests/unit/router_utils/test_reasoning_effort_capability.py
+++ b/tests/unit/router_utils/test_reasoning_effort_capability.py
@@ -150,12 +150,12 @@ class TestNoneLevelPolarity:
 
     def test_none_stays_opt_out_on_an_azure_model_outside_the_gpt_5_family(self):
         """AzureOpenAIGPT5Config is selected by is_model_gpt_5_model, so an azure o-series or
-        anthropic deployment never reaches the gate that refuses none and must keep the level."""
+        anthropic deployment never reaches the gate that refuses none unless its row turns none off."""
         resolved = resolve_supported_reasoning_efforts(
             {
                 "supports_reasoning": True,
                 "litellm_provider": "azure",
-                "key": "azure/o3",
+                "key": "azure/o-series-deployment",
                 "supports_max_reasoning_effort": True,
             },
             deployment_is_mapped=True,