Can GPT-OSS 20B run on Mac Studio M3 Ultra 256GB?

YES — Runs Great

S86Excellent

Estimated from fit model

GPT-OSS 20B needs ~43.8 GB VRAM. Mac Studio M3 Ultra 256GB has 184.3 GB. With Q4_K_M quantization, expect ~100 tok/s.

Runtime: llama.cppCapacity: RoomyBandwidth: HighStack: StandardBottleneck: Balanced

Operating mode

Choose the run profile you care about

Interactive favors responsiveness, while light API and scale-out lean harder on serving readiness. The fit stays the same, but the recommendation lens changes.

Current mode

Balanced

Balanced for general local use. Keeps the ranking neutral across personal and serving workflows.

Capabilities:

Select quantization to explore

Q4_K_M (Medium quality) — 43.8 GB, 106.9 tok/s, Runs well

43.8 GB required184.3 GB available

24% VRAM used

Fit status

Runs well

Decode

106.9 tok/s

TTFT

1811 ms

Safe context

128K

Memory

43.8 GB / 184.3 GB

Memory breakdown

Weights12.8 GB

KV Cache2.4 GB

Runtime0.9 GB

Headroom27.6 GB

See how fast it feels

See how fast it feelsGPT-OSS 20B on Mac Studio M3 Ultra 256GB
1st promptCold start — includes initialization
>Write a Python function to sort a list
Here's a Python function that sorts a list using quicksort:

def quicksort(arr):
    if len(arr) <= 1:
        return arr
    pivot = arr[len(arr) // 2]
    left   = [x for x in arr if x < pivot]
    middle = [x for x in arr if x == pivot]
    right  = [x for x in arr if x > pivot]
    return quicksort(left) + middle + quicksort(right)

# Example usage
numbers = [3, 6, 8, 10, 1, 2, 1]
print(quicksort(numbers))  # [1, 1, 2, 3, 6, 8, 10]

The algorithm picks a pivot, partitions the list into smaller, equal, and larger elements, then recursively sorts each partition and concatenates the results.
2nd promptWarm — model ready, no init overhead
>Now add type hints and a docstring
def quicksort(arr: list[int]) -> list[int]:
    """Sort a list using the quicksort algorithm.

    Args:
        arr: List of integers to sort.

    Returns:
        A new sorted list.
    """
    if len(arr) <= 1:
        return arr
    pivot = arr[len(arr) // 2]
    left   = [x for x in arr if x < pivot]
    middle = [x for x in arr if x == pivot]
    right  = [x for x in arr if x > pivot]
    return quicksort(left) + middle + quicksort(right)

Added type hints for the input and return type, plus a Google-style docstring.
Estimated: 106.9 tok/s decode · 1.8s TTFT (warm) · 267 tok/s prefill

What limits this setup

This setup is broadly balanced for this model.

Shared-memory contention still exists

The OS, browser, and inference runtime all compete for the same physical memory pool, so real-world headroom is less forgiving than raw capacity suggests.

Best improvement path

Performance by workload

Workload	Grade	Fit	Decode	TTFT	Context
Chat	S	Runs well	99.5 tok/s	1062 ms	128K
Coding	S	Runs well	99.5 tok/s	1947 ms	128K
Agentic Coding	S	Runs well	99.5 tok/s	2831 ms	128K
Reasoning	S	Runs well	99.5 tok/s	2301 ms	128K
RAG	S	Runs well	99.5 tok/s	3539 ms	128K

Quantization options

How GPT-OSS 20B (21B params) fits at each quantization level on Mac Studio M3 Ultra 256GB (184.3 GB usable).

Quant	Bits	VRAM	Quality	Fit
Q2_K	2	8.2 GB	Low	A76
Q3_K_S	3	10.3 GB	Low	A76
NVFP4	4	11.8 GB	Medium	A76
Q4_K_M	4	12.8 GB	Medium	A76
Q5_K_M	5	15.1 GB	High	A76
Q6_K	6	17.2 GB	High	A76
Q8_0	8	22.5 GB	Very High	A76
F16Best for your GPU	16	43.1 GB	Maximum	A79

Get started

Copy-paste commands to run GPT-OSS 20B on your machine.

Run

ollama run gpt-oss

Your hardware

More models your Mac Studio M3 Ultra 256GB can run

Model	Params	Grade	Decode
Devstral 2 123B Instruct	123B	S	8.1 tok/s
Qwen3-Coder 30B A3B Instruct	30.5B	S	84.2 tok/s
Qwen 3.5 27B	27B	S	36.5 tok/s
Qwen 3.6 27B	27B	S	27.8 tok/s
Qwen 3.5 122B A10B	122B	S	34.7 tok/s

Frequently asked questions

Can Mac Studio M3 Ultra 256GB run GPT-OSS 20B?

Yes, Mac Studio M3 Ultra 256GB can run GPT-OSS 20B with a S grade (Runs well). Expected decode speed: 99.5 tok/s.

How much VRAM does GPT-OSS 20B need?

GPT-OSS 20B (21B parameters) requires approximately 43.8 GB of memory with Q4_K_M quantization.

What is the best quantization for GPT-OSS 20B?

The recommended quantization for GPT-OSS 20B is Q4_K_M, which balances quality and memory efficiency.

What speed will GPT-OSS 20B run at on Mac Studio M3 Ultra 256GB?

On Mac Studio M3 Ultra 256GB, GPT-OSS 20B achieves approximately 99.5 tokens per second decode speed with a time-to-first-token of 1947ms using Q4_K_M quantization.

Can Mac Studio M3 Ultra 256GB run GPT-OSS 20B for coding?

For coding workloads, GPT-OSS 20B on Mac Studio M3 Ultra 256GB receives a S grade with 99.5 tok/s and 128K context.

What context window can GPT-OSS 20B use on Mac Studio M3 Ultra 256GB?

On Mac Studio M3 Ultra 256GB, GPT-OSS 20B can safely use up to 128K tokens of context. The model's official context limit is 128K, but available memory constrains the safe maximum.

Is unified memory on Mac Studio M3 Ultra 256GB as fast as VRAM for GPT-OSS 20B?

Not always. Mac Studio M3 Ultra 256GB can often fit larger models thanks to unified memory, but a discrete GPU with dedicated high-bandwidth VRAM may still decode faster once the model fits. For this combination, the important distinction is capacity versus sustained throughput.

See all results for Mac Studio M3 Ultra 256GB See all hardware for GPT-OSS 20B

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