Adam Brody and Rachel Bilson dated for almost the entire run of the show (2003-2006). Additionally, several inside jokes are written into the show about this.
The infamous "Welcome to the O.C., bitch!" was not a line in the original script.
When Ben McKenzie was cast, he was working as a telemarketer.
There is now a Sandy Cohen Public Defender Fellowship at Berkeley Law School. Students commonly refer to it as "The Sandy" or "The Sandy Cohen Public Defender Fellowship for Lawyers Who Dare to Dream."
In one scene the characters talk about Seth (played by Adam Brody) vomiting "like that girl from 'The Sixth Sense'". A joke, since Mischa Barton played that very girl in the film The Sixth Sense (1999).