Original: Mac OS X is based upon the Mach kernel . Certain parts from FreeBSD 's and NetBSD 's implementation of Unix were incorporated in NeXTSTEP , the core of Mac OS X. NeXTSTEP was the object-oriented operating system developed by Steve Jobs ' company NeXT after he left Apple in 1985 .
Simplified: Mac OS X is a UNIX OS that is based on NEXTSTEP , an older OS that Apple bought and turned into Mac OS X. Mac OS X and NEXTSTEP have a background in a kind of UNIX called BSD ( more specifically FreeBSD and NetBSD ) .