It's a bit of an amalgam, and it honestly depends on which version of OSX you're talking about. Mach itself was designed as an improvement over BSD, and it was more or less a microkernel developed around BSD concepts with binary compatibility. My understanding is that as time went on, performance issues and security concerns forced Apple to poach more and more BSD code onto Mach, which leaves us with the pretty messy codebase they use today. Supposedly (?) a lot of the zombie code is being cleaned up internally, but it's been an ongoing, multi-year process for Apple, probably made even more difficult with their bonafide UNIX certification.
Also, random thing, but both Orbis (Playstation operating system) and OS X are based on FreeBSD.