According the the genealogy from this Wikipedia article OS X came from BSD, so again, why is it closed source and licensed as proprietary? But, the Darwin OS, also from Apple is open sourced?
First, being certified as a Unix says nothing about your licensing, just your compliance to the established standards for being Unix.
Second, because being Unix has nothing to do with your licensing, and everything to do with your being like Unix, an originally proprietary system, and one with a long legacy.
Finally, don't mistake licensing: BSD code can subsequently be closed, and there is no duty of disclosure or source availability. The BSD license is more free on one axis, and less free on another, because of that.
If a system is certified as Unix, that system's vendor is allowed to call it Unix. That is precisely what the certification is for.
Systems such as GNU/Linux and *BSD, which look and behave like Unix, but are not actually certified as Unix, are called Unix-like. Since the various open source projects have limited finances, they generally want to spend their money on development and not certification, so don't expect the status quo to change greatly any time soon.
Apple did Open Source a good portion of the underlying, BSD based, pieces of OSX. See Wikipedia's Darwin Entry for full details as well as a release history. Apple keeps the upper level stuff proprietary, like their UX components and does not release those as OSS. If you want a full list of all the Apple OSS projects/inclusions, check out Apple's Open Source page.
OpenBSD and FreeBSD is licensed under the BSD License which says:
Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution
This explicitly allows one to distribute a modified binary without delivering source.
Apple has released the source code for Darwin and some parts of OS X.