Do you need both definitions of OK?
If not, something like this might work
Code:
#define OK BAH_HUMBUG
#include "common.h"
#undef OK
#include "libsomething.h"
If you do need both, then you'll have to create your own small wrapper API which results in you only #including one of the header files at once.
That can get a bit messy if you're not careful.
If you only want say a small part of libsomething.h (say), and most of common.h, then you might also be able to just wrap the parts of libsomething which really interest you.
Also, raise a bug report on both packages telling them about the poor use of generic names like "OK", and the likely conflicts it generates.