Actually the mac OS is built in part with freeBSD. Open source nix has always had a place at Apple, but Apple greedy and bad right?

http://en.wikipedia.org/wiki/BSD

Darwin, the system on which Apple's Mac OS X is built, is a derivative of 4.4BSD-Lite2 and FreeBSD

Here’s a good breakdown: https://stackoverflow.com/a/3449195

The Wikipedia BSD article is good (and accords with my own understanding, for what that's worth). It says that Darwin, the system on which Apple's Mac OS X is built, is a derivative of 4.4BSD-Lite2 and FreeBSD, and notes that 4.4BSD is the last release that Berkeley was involved with.

So, Darwin is as BSD as you can get (just like all the other BSDs!). OS X refers to those parts of the distribution which aren't open-source, principally the GUI, but including a variety of frameworks, and anything which relies on these won't be portable.

OS X as a whole is a UNIX 03 system. That's equivalent to being a truly POSIX-compliant system (as opposed to being POSIX-like).

As other answers have noted, the userland parts of the OS are unsurprising to anyone with much unix experience, and I've rarely had any difficulty building portable-unix software on OS X.

In contrast, the non-userland parts of the OS are pretty different. Apple seems to be willing to innovate in those areas fairly cheerfully. I think (but I'm not positive) that these changes are formally part of Darwin. One of the most obvious differences is that launchd has replaced cron, at, inetd, and much of the startup infrastructure.

….But go ahead and tell us how mac and open source is antithetical

Edit: for the downvoters, yes Apple has had their hands in open source from the start and never stopped. Doesn’t mean they’re the goddamn FOSS Jesus

Open source software is at the heart of Apple platforms and developer tools. Apple manages the following projects and encourages your contribution.

https://opensource.apple.com/projects/