For a long time, I've been under the impression, largely from friends who have been running Macintoshes for ages, that OS-X was based on one of the three BSD forks - that essentially it was a *BSD with an Apple window manager/desktop environment, and with the kernel (and other programs) locked to the Trusted Program Module (TPM).
Comments from some of you lately led me to do some research. It appears that, at least on the internet, current documentation agrees that OS-X is based on Openstep, which evolved from NeXTSTEP, which was derived from 4.4 BSD (pre-fork?) and the Mach OS.
It's funny how knowledge like this can evolve on the web. What gets collated, analyzed, and archived becomes the truth. What was known to be true at the time gets forgotten. Mode Emulators become Modulator/Demodulators. Other acronyms drift.