It looks like I got oo2c to build and work under Mac OS X Snow Leopard, with some stupid hacks, at least enough for my purposes. I don't claim that this is suitable for production work.
Here's the configure invocation I used:
CFLAGS='-no-cpp-precomp -fnested-functions -I/opt/local/include' \ LDFLAGS='-L/opt/local/lib' \ ./configure --prefix=/sw/versions/m64/oo2c/2.1.11 --disable-libs
Here's the patch.