It depends on who you ask and what you consider "Unix." Unix, in it's most proper form, is an OS that has passed a bunch of compliance tests. The most important of which are POSIX and ISO C compliance. To my knowledge no distro of Linux, BSD, or OS X has passed any of the variations of Unix certification. I don't think any have even tried.
Linux, BSD, and OS X all are somewhat POSIX compliant, though they may not pass the entire battery of tests. So from a standpoint of use and utility OS X, BSD, and Linux are considered Unix style OSes. It does not mean they are a flavor of Unix though. Look at Apple's propoganda and you will see what I mean. "Unix based" not "Unix."
This is why you will see some people refer to Linux, BSD, OS X, et al. as *nix. Not technically Unix but close enough for most purposes.