o1

o1 - The First Full Public o-Series Model with an Internal COT

o1 is the foundational full-scale model in OpenAI's o-series. It is trained with reinforcement learning to produce a long internal chain of thought before generating a response. This pre-answer reasoning step is the defining mechanism of the o-series line and was first delivered at full scale through o1.

How o1 Differs from the o-Series That Followed

o1 established the o-series pattern: RL-trained chain-of-thought reasoning before each answer, slowest speed tier, no fine-tuning support, and no predicted outputs. Later models in the series (o3, o3-pro) carry forward this pattern with additional capabilities such as image integration into the reasoning chain and RL-trained tool use. o1 does not include those extensions.