To build OccJava with mingw you need a OpenCASCADE binaries built with mingw. Official binaries are built with Microsoft Visual C++.

The follwing patches are needed to build OpenCASCADE 6.5 with mingw:
- patch from the Opencascade Debian package (quilt --quiltrc debian/patches/quiltrc push -a)
- occ-mingw-makefile.patch
- occ-mingw-port.patch

Once OpenCASCADE 6.5 is built with mingw you can build OccJava by setting CASROOT and JAVA variables in Makefile.mingw and run make -f Makefile.mingw.

To build with mingw on Debian you need mingw32 mingw32-binutils mingw32-runtime packages.

All these has only been tested on Debian but should also work with other Linux distros and MSYS.