Greg Brockman appears to confirm o1, o3 and GPT-5 all built on the GPT-4o base model
flowersslop · x · 2026-09-03
Greg Brockman seems to confirm what the author long suspected: OpenAI's o1, o3, and GPT-5 Instant/Thinking were all built on the GPT-4o base model. The original o1 was internally called "GPT-4o with reasoning," suggesting OpenAI's reasoning series advanced via inference-layer iteration rather than new base models.
Related event: Brockman Suggests o1/o3 and GPT-5 Are Built on GPT-4o(2 posts)→
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