Currently the OCC/DFOCC page of the docs makes no mention of frozen core.
DFOCC seems to default to all-electron, but does seem to run with set freeze_core true.
OCC throws a RuntimeError, and claims that "Frozen core/virtual not implemented in Orbital-optimized methods"

I can see two issues here:

1. The error message from OCC suggests that none of the orbital-optimized methods in Psi4 support FC. This is either wrong and misleading (if DFOCC really does support FC) or DFOCC should also throw the same error.
2. Regardless of the answer to 1, the relevant doc page ought to have a few words about FC support or the lack thereof.