OpenAI o1 is the full release of OpenAI's first-generation reasoning model, launched in December 2024. It excels at complex problem-solving in science, coding, and math by thinking before it answers, utilizing a 'chain of thought' process. While significantly slower than standard models due to this reasoning step, it achieves superior performance on difficult benchmarks like GPQA and AIME. It supports multimodal inputs and tool calling, making it a powerful engine for deep analytical tasks.