Overview
gpt-oss-120b is an open-weight language model from OpenAI, released under the Apache-2.0 license. It uses a Mixture-of-Experts (MoE) design: of its ~120B total parameters, only about 5B are active per token, which is what lets a model this large run on a single workstation-class GPU. It handles a 128K-token context window and ships with built-in reasoning, tool calling, and code capabilities.
Because the weights are public, you download the model once and run every prompt on your own machine inside Atomic Chat. Nothing leaves the device, there is no API key, and the model keeps working with the network turned off. That makes gpt-oss-120b a fit for private documents, regulated environments, and offline use.
What it is good at
The capability tags on gpt-oss-120b are tools, thinking, reasoning, and code. Those map onto a few concrete jobs:
- Agentic tool use — native function calling lets the model trigger searches, run code, or hit local tools and fold the results back into its answer.
- Step-by-step reasoning — it exposes its chain of thought and supports adjustable reasoning depth, useful for math, planning, and multi-step problems. OpenAI reports it lands near o4-mini on core reasoning benchmarks.
- Coding help — writing, explaining, and debugging code, including longer files that benefit from the 128K context.
Running it locally
gpt-oss-120b reports a 120.4B parameter count, and OpenAI ships the checkpoint in MXFP4 form at roughly 61 GiB. The clean target for local serving is a single 80 GB GPU such as an H100; setups with less VRAM can run it through CPU offloading at lower speed. The 128K context window means you can feed it large documents or long chat histories.
huggingface-cli download openai/gpt-oss-120b
From there you can serve it with Transformers or vLLM, or load it in Atomic Chat with one click and start chatting fully on-device.
License
gpt-oss-120b is released under Apache-2.0. You can use it commercially, modify it, fine-tune it, and ship products built on it, with no copyleft requirement and an explicit patent grant. Keeping the license notice is the main obligation.