Do you need both definitions of OK?
If not, something like this might work
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.
#define OK BAH_HUMBUG
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.