IIRC Mac OS uses a completely different kernel to FreeBSD - it's a heavily modified Mach microkernel derivative and FreeBSD's is a traditional Unix monolithic kernel. Mac OS does use the 'user-land' from BSD that runs on top of the kernel. The user-land layer is the Unix-y bit i.e. shells etc that run in a less-priviledged mode on the CPU than the kernel. Mac OS X also uses BSD's virtual file system and networking stack code.