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OS X is not "FreeBSD" underneath. The userspace comes from FreeBSD partially, but many components of the userspace come directly from NeXT (i.e. NetInfo). OS X does not use BSD-style init either.
As for the kernel, OS X's process scheduler and VMM, which I would consider the two most important components of an operating system kernel, come directly from Mach. While FreeBSD's VMM is a descendant of Mach, it has been *heavily* modified over the past decade.
The components of the kernel that do come from FreeBSD are the unified buffer cache, the VFS, and the networking components. I would assume that the asynchronous I/O implementation and kqueue implementation are largely based on FreeBSD as well.
The bottom line is that Darwin is a combination of NeXT and FreeBSD components and XNU is a hybrid Mach/FreeBSD kernel, with the important components of the kernel being Mach, and the FreeBSD components being relatively uninteresting.