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 ) . The core of Mac OS X is an open source OS called Darwin , but Darwin can not run Mac OS X software . Carbon was not included in the first product sold as Mac OS X , Mac OS X Server ( now known as Mac OS X Server 1 .