Opengl .obj

.obj files must be linked into your project during the building process. Im afraid that''s not something you can do at runtime.

Use your favorite linker and just link it in.

If you really want to do it dynamically, look into dynamic libraries (don''t know much about these on OS X).

I think the OP is talking about Wavefront Object files, not the obj files beeing produced by a compiler.
The format description can be found here.