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.
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 El sexto sentido (1999).
When Adam Brody tested for the role of Seth Cohen, he decided to improvise everything. That didn't sit well with writer Josh Schwartz, who told the producers afterward that he never wanted "to see that kid ever again". A month later, when they still had not found the right Seth, the producers called Brody back for another test. Despite Schwartz not liking the fact that Brody would not stick to the writing, he had to admit that there was something about him. Schwartz later confessed that Brody reminds him of himself in many ways - which in the end was perfect for the role of Seth, since it is based on the writer himself.