Mac OS X is unix based. When you install it, (up to 10.3 panther I believe, the other installers don't mention it) it says during the installation "UNIX Based"

aside from that, it is on apple's webpage

http://osxbook.com/book/bonus/ancient/w ... /arch.html
It's a little bit old article, but in fact it's kinda the same thing..

Well...

To my understanding Apple took the original Unix code, and made an completely independent OS.
You can install the X11 interface and compile Linux based applications on it. So in a sense OS X is Linux, and still it is not.

Mac OS X is based on a Unix Kernel

While Mac OS X is indeed a distant and mixed variant of several BSDs (well mainly OpenBSD, but still), everything above kernel level is completely different from most Unix or Unix-based system. I'm suprised Mac OS X is UNIX certified while Linux isn't.

Linux is the kernel in a GNU/Linux operating system. This is casually referred to as Linux.
Darwin is modified kernel taking code from NEXTSTEP, BSD, and UNIX.

As silly as it is, this old copypasta describes it well:
"I'd just like to interject for a moment. What you're referring to as Linux, is in fact, GNU/Linux, or as I've recently taken to calling it, GNU plus Linux. Linux is not an operating system unto itself, but rather another free component of a fully functioning GNU system made useful by the GNU corelibs, shell utilities and vital system components comprising a full OS as defined by POSIX.

Many computer users run a modified version of the GNU system every day, without realizing it. Through a peculiar turn of events, the version of GNU which is widely used today is often called Linux, and many of its users are not aware that it is basically the GNU system, developed by the GNU Project.

There really is a Linux, and these people are using it, but it is just a part of the system they use. Linux is the kernel: the program in the system that allocates the machine's resources to the other programs that you run. The kernel is an essential part of an operating system, but useless by itself; it can only function in the context of a complete operating system. Linux is normally used in combination with the GNU operating system: the whole system is basically GNU with Linux added, or GNU/Linux. All the so-called Linux distributions are really distributions of GNU/Linux."

I belive all OS's are built off of Linux just added their own UI overhaul. Linux isent only a specific OS its also code itself...