[TZubiri]

https://github.com/openai/gpt-oss/tree/main

Sure a project can be based on more than 1 language. But it seems to be mostly python.

[pjmlp]

You missed these trees on the Python forest,

https://github.com/openai/gpt-oss/blob/main/pyproject.toml

https://github.com/openai/gpt-oss/blob/main/CMakeLists.txt

[TZubiri]

You are right, they are using Docker, therefore it's written in C.