The UNIX trademark is owned by The Open Group, which basically uses it as a certification mark for SUS-compliance. OS X has been registered as a UNIX 03-compliant product since 10.5, so it qualifies as UNIX (not just UN*X or Unix-like) in that sense. Most GNU/Linux distributions are not registered as SUS-compliant, so they should be called something like UNIX-like or UN*X in that sense, even though neither of those terms is used by The Open Group.
OS X could also be considered to be genetic Unix in the sense that Darwin was based on FreeBSD (or it has used FreeBSD as a reference platform), and BSDs are usually considered to be genetic Unix, since the original BSD was based on AT&T code. Both GNU and Linux were rewritten from scratch though, so GNU/Linux platforms are not generally considered to be genetic Unix.
In a functional or spiritual sense, OS X is maybe less Unix (whatever that means) than platforms that are merely "Unix-like" in the first or second senses, but OS X could still considered to "be Unix" in all three senses.