OpenAI o

OpenAI o1 is a reasoning model that thinks before it answers. Trained with large-scale reinforcement learning, it uses a hidden chain of thought to break down tricky problems, correct its own mistakes, and try new strategies when one isn't working. Released as an early preview in September 2024, it was built for professionals who need verifiable accuracy over raw speed—competitive programmers, researchers in physics and chemistry, and developers integrating reasoning into their apps. In benchmarks, o1 scored in the 89th percentile on Codeforces, averaged 74% on the 2024 AIME math exam with a single sample, and surpassed PhD-level accuracy on GPQA-diamond (chemistry, physics, biology). With vision enabled, it hit 78.2% on MMMU and beat GPT-4o on 54 of 57 MMLU subcategories. Performance scales with test-time compute: more thinking time means better results, and you can push accuracy further with majority vote across 64 samples or re-ranking 1000 samples with a learned scoring function. What sets o1 apart is its ability to handle multi-step, reasoning-heavy tasks where GPT-4o and earlier models plateau. It's not a general-purpose chatbot—it's a specialized workhorse for problems that reward deliberate, step-by-step reasoning. The trade-off is slower responses and higher computational cost, which is why it's best suited for API use cases that can wait for a thorough answer. As of 2026, o1 has been superseded in ChatGPT by newer models like GPT-5.6 Luna and GPT-5.6 Sol, so it's no longer the default for everyday users. OpenAI has also introduced Ultrafast mode for GPT-5.6 Sol, delivering up to 14x faster inference. For developers who need deterministic, high-accuracy reasoning via the API, o1 remains a viable option—especially when you can architect around its deliberate pace.