o-series models
Also known as: o1, o3, o4-mini, o4, OpenAI o-series, reasoning model series
OpenAI's o-series launched with o1 in late 2024 and established a distinct model family philosophy: instead of optimizing purely for speed and fluency, these models allocate significant compute to internal reasoning before producing output. They generate a chain of thought (invisible to users by default) and only then produce a final answer.
o3 and o4-mini were released in April 2025 as the second generation, with o3 setting state-of-the-art results on SWE-bench and multiple science benchmarks. o4-mini is the cost-efficient version optimized for developer workloads where you want reasoning capability without the cost of the full o3 model. By late 2025, reasoning capabilities began merging into GPT-series flagship models, blurring the line between model families.
For builders, the o-series models are most valuable for tasks where the reasoning process matters: debugging complex code, analyzing a legal document, working through a multi-step math problem, or any scenario where a first-pass answer is usually wrong and the model benefits from self-correction. For creative tasks, rapid generation, or simple lookups, they're usually overkill.