[Frumious Bandersnatch]

Sort of. I believe that Mac is actually built on the Mach kernel which is a BSD derivant (whether freeBSD, OpenBSD or just plain BSD, I don’t know).

[AnotherUnixGeek]

I believe that Mac is actually built on the Mach kernel which is a BSD derivant (whether freeBSD, OpenBSD or just plain BSD, I don’t know).

Mach predates the FreeBSD/OpenBSD/NetBSD projects, and in the late '80s incorporated most of 4.3 BSD at one point. This might be what you're referring to. MacOS X/XNU uses a heavily modified version of Mach 3.0, with the modifications including the contributions specifically from the FreeBSD OS which I listed above.