o1 ist most likely just 4o optimized for CoT with some fine tuning or perhaps merely with a dedicated system prompt (which is probably the reason why they don't

o1 ist most likely just 4o optimized for CoT with some fine tuning or perhaps merely with a dedicated system prompt (which is probably the reason why they don't let you access it in the API) and enforced structured output. In fact you can recreate something very similar using 4o and the right system prompt + structured outputs.