I'm from California, and no one who lives in Orange County calls it "the O.C." If they do, they are mindless sheep who spend too much time fantasizing about TV "reality".

The closest i've ever heard a native call it is "OC" without the "the" in front, and then usually only when using some sort of acronym, like OCCC, for Orange County Community College, etc.

As far as where to ride, try this link, has some good info:
http://www.apc.net/maross/ocmtb.htm