o1, released December 5, 2024 after a September preview, was the first production reasoning model: instead of answering immediately, it generates a long private chain of thought trained with reinforcement learning before responding. The approach produced dramatic gains on competition mathematics, science, and hard coding problems that had resisted ordinary scaling.

o1 established the test-time-compute paradigm that reshaped the entire industry — within months every major lab shipped a reasoning model, from DeepSeek R1 to Gemini 2.5 Pro to Claude 3.7 Sonnet's extended thinking. Its own line moved fast too: o3-mini arrived in January 2025 and the full o3 in April 2025.