OpenAI o

OpenAI o1 was a landmark in reasoning—reinforcement learning taught it to think before speaking, achieving PhD-level accuracy on hard science benchmarks. In practice, it made math, coding, and logic problems solvable by AI with unprecedented precision. But it was never a general-purpose chatbot: it's slow, verbose in thought, and the hidden reasoning chain can frustrate users who want transparency. Today (mid-2026), o1 has been eclipsed by GPT-5.5 Thinking and GPT-5.5 Pro, which incorporate o1's reasoning advances into faster, more capable models. OpenAI's current ChatGPT plans no longer mention o1; instead, Plus users get GPT-5.5 Thinking and Pro users get GPT-5.5 Pro. The original o1 API remains available for developers who need the specific chain-of-thought behavior or who have fine-tuned on o1 outputs. Where o1 still shines is as a reference for reproducible reasoning: its performance on AIME (93% with 1000 samples) and GPQA-diamond remains impressive, and some researchers prefer its output style for auditing. However, for most buyers, choosing GPT-5.5 Thinking or Pro is simpler—they offer comparable or better reasoning plus lower latency, vision, and agent mode. The main caveat: o1's $200/mo Pro tier from 2024 is gone; now you pay for GPT-5.5 Pro at the same price. If you need the exact o1 model for reproducibility or compatibility, the API is your path. Otherwise, skip o1 and go with the newer models.