[gnfurlong]

Wasn't OSX originally a fork of FreeBSD? Parts of it may have been replaced over time, but I would think that a large portion is still written in C.

[platinumrad]

OS X is basically the XNU kernel (written mostly in C) + the FreeBSD userland (written mostly in C) + a few GNU utilities (written mostly in C) + a bunch of Apple stuff like iTunes, Safari, etc. (Written mostly in C++ and Objective-C). It's not really a "fork" of FreeBSD.

[pjmlp]

NeXTSTEP drivers were written in Objective-C, and OS X drivers are written in a C++ subset.

[kbenson]

I think the terminal includes a lot of GNU utilities, so that's likely true. That doesn't actually conflict with anything I said though. I'm sure the GUI code is quite large, given that the system is meant to be maintained and usable from entirely within that context.