benchlm.ai

GPT-5.3 Codex vs GPT-OSS 20B

Side-by-side benchmark comparison across knowledge, coding, math, and reasoning.

Quick Verdict

GPT-5.3 Codex wins overall with a score of 85 vs 22 (63 point difference).GPT-5.3 Codex wins 4 out of 4 categories.

GPT-5.3 Codex

CreatorOpenAI
TypeProprietary
Context400K
Score85

GPT-OSS 20B

CreatorOpenAI
TypeOpen Weight
Context128K
Score22

Knowledge

GPT-5.3 Codex

GPT-5.3 Codex

96

GPT-OSS 20B

28.8

99
MMLU
31
97
GPQA
30
95
SuperGPQA
28
93
OpenBookQA
26

Coding

GPT-5.3 Codex

GPT-5.3 Codex

95

GPT-OSS 20B

23

95
HumanEval
23

Mathematics

GPT-5.3 Codex

GPT-5.3 Codex

97.1

GPT-OSS 20B

30

99
AIME 2023
31
99
AIME 2024
33
98
AIME 2025
32
95
HMMT Feb 2023
27
97
HMMT Feb 2024
29
96
HMMT Feb 2025
28
96
BRUMO 2025
30

Reasoning

GPT-5.3 Codex

GPT-5.3 Codex

94

GPT-OSS 20B

28

95
SimpleQA
29
93
MuSR
27

Frequently Asked Questions

Which is better, GPT-5.3 Codex or GPT-OSS 20B?

GPT-5.3 Codex scores higher overall with 85 vs 22, a difference of 63 points across all benchmarks.

Which is better for knowledge tasks, GPT-5.3 Codex or GPT-OSS 20B?

GPT-5.3 Codex leads in knowledge tasks with an average score of 96 vs 28.8.

Which is better for coding, GPT-5.3 Codex or GPT-OSS 20B?

GPT-5.3 Codex leads in coding with an average score of 95 vs 23.

Which is better for math, GPT-5.3 Codex or GPT-OSS 20B?

GPT-5.3 Codex leads in math with an average score of 97.1 vs 30.

Which is better for reasoning, GPT-5.3 Codex or GPT-OSS 20B?

GPT-5.3 Codex leads in reasoning with an average score of 94 vs 28.

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