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.
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).
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.