I'm aware of https://github.com/orafce/orafce and https://github.com/pgoracle before that (the commit history also references ora2pg and searching for that surfaces https://github.com/darold/ora2pg which was just updated yesterday!) but I've never tried to use any of them in anger
Perhaps you could directly use or decompile the OCI libs. Though that probably is against some license.
I'm aware of https://github.com/orafce/orafce and https://github.com/pgoracle before that (the commit history also references ora2pg and searching for that surfaces https://github.com/darold/ora2pg which was just updated yesterday!) but I've never tried to use any of them in anger