if you look through their drivers, they use a sort of c++ without exceptions, but last i remember from debugging network drivers on osx, they looked like they were forked from freebsd a fairly long time ago, but were for a long time close enough to be able to see the roots(i don't know if it's still like that)
I occasionally get caught by a difference between the BSD and GNU userlands so I've often contemplated giving one of the BSDs a go rather than my typical OS X/Debian/Ubuntu Dev/prod web server preference.