GPT-OSS-120B is OpenAI's first open-weight language model, featuring 117 billion total parameters in a Mixture-of-Experts architecture that activates just 5.1 billion per forward pass. Optimized to run on a single 80GB GPU with native MXFP4 quantization, it achieves near-parity with o4-mini on core reasoning benchmarks while supporting configurable reasoning depth, full chain-of-thought access, and native tool use including function calling and structured outputs. Released under the Apache 2.0 license, it brings frontier-level reasoning and agentic capabilities to a fully customizable, locally deployable model.