OpenAI o3 and o4-mini support Flex processing: this feature provides significantly lower costs for Chat Completions or Responses requests in exchange for slower response times and occasional resource unavailability.

To take advantage of Flex processing, you have to set the "service_tier" parameter to "flex" in your API request (Chat or Responses).

Reference: https://platform.openai.com/docs/guides/flex-processing?api-mode=responses