Nobody wants to use a model with a "FOSS" - as in copyleft - license, since you could conceivably need to license anything it's deeply integrated in under a compatible license. The gpt-oss README specifically mentions the lack of "copyleft restrictions or patent risk" as a selling point.

FOSS does not imply copyleft. Permissive licenses and copyleft licenses are both FOSS.

They're Apache licensed.