Home / Can I run it? / gpt-oss-120b / gpt-oss-120b on a Mac Studio 256GB

Yes. gpt-oss-120b at Q8_0 needs about 143.8 GB, and the Mac Studio M5 Ultra 256GB has 192.0 GB usable at a ~4k context.

Can I run gpt-oss-120b on a Mac Studio 256GB?

gpt-oss-120b at Q8_0 needs about 143.8 GB, and the Mac Studio M5 Ultra 256GB has 192.0 GB usable. That figure is 256 GB of unified memory on the spec sheet, of which 192.0 GB is usable for weights and cache, at 1200 GB/s memory bandwidth. OpenAI’s open 120B-class gpt-oss model. It needs large unified memory at Q4, with a much smaller active set per token. These are estimates from our formula, not measured benchmarks.

Quant that fits

Q8_0 Fits with headroom

Memory needed

143.8 GB at Q8_0, ~4k context

Usable unified memory

192.0 GB of 256 GB unified memory

Est. tokens/sec

up to 177 estimate

gpt-oss-120b · Mac Studio M5 Ultra 256GB · Everything this machine runs · Q4 vs Q8 vs FP16

Memory at each quant

Estimated tokens per second

Estimate: up to 177 tokens/sec at Q8_0 (upper bound: this is a Mixture-of-Experts, and only the active weights are counted). This is a bandwidth-derived range, not a bench. Efficiency is a 45–80% unified-memory band from How we calculate.

Context headroom

At Q8_0, this machine has headroom out to about 128k tokens under our KV estimate. Longer chats need more memory for the cache. Figures assume an FP16 KV cache.

From the model’s config. Try other sizes in the VRAM calculator or see the context length guide.

What to do instead

• See the best GPU for this model family and the gpt-oss-120b model page.

Other hardware for gpt-oss-120b

• gpt-oss-120b on an RTX 4090Won’t fit
• gpt-oss-120b on an RTX 5090Won’t fit
• gpt-oss-120b on an RTX 3090Won’t fit
• gpt-oss-120b on dual RTX 3090sWon’t fit
• gpt-oss-120b on a Mac mini 64GBWon’t fit
• gpt-oss-120b on a Mac Studio 512GBFits

Other models on a Mac Studio 256GB

• Llama 3.3 70B on a Mac Studio 256GBFits
• R1 Distill 70B on a Mac Studio 256GBFits
• Qwen3-235B on a Mac Studio 256GBFits
• DeepSeek-R1 on a Mac Studio 256GBWon’t fit
• DeepSeek-V3.2 on a Mac Studio 256GBWon’t fit
• Mixtral 8x22B on a Mac Studio 256GBTight