o3, released April 16, 2025 alongside o4-mini, was OpenAI's most capable reasoning model of the o-series era and the first to combine extended reasoning with full agentic tool use in ChatGPT — searching the web, running Python, and analysing images inside its chain of thought. First previewed in December 2024 with breakthrough ARC-AGI results, it took four months to reach general availability.

In practice o3 became the model of choice for hard analytical work — mathematics, research questions, multi-step debugging — while GPT-4.1 handled everyday tasks. o3-pro followed in June 2025 for even longer reasoning budgets. The separate o-series branding ended when GPT-5 unified fast and reasoning models under one system in August 2025.