GPT-OSS 120B is a 116.83B-parameter mixture-of-experts model (5.1B active per token). At MXFP4 it needs about 64.1 GB of memory. The Mac Studio M5 Ultra 256GB has 192 GB usable, so it fits entirely in memory. Expect about 111 tokens/sec (99–123), with the first token after 0.3 s for a short prompt. This estimate is based on community benchmarks of this exact setup.
| Quantization | File size | Memory | Generation | First token | Source |
|---|---|---|---|---|---|
| MXFP4 | 63.4 GB | Fits | 111 tok/s | 0.3 s | Measured |
All models on the Mac Studio M5 Ultra 256GB · GPT-OSS 120B on all hardware
Estimates for a short prompt on a single device. Measured numbers come from community benchmarks (r/LocalLLaMA, localscore.ai and more); others are derived from similar setups or projected from specs. Open source on GitHub.