Melinda Clarke and Rachel Bilson were only meant to be guest stars, but their characters became so important and integral to the show's popularity that they finally got signed as regulars and appeared in the opening credits from the last half of season 1 and on.
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).
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.
Adam Brody (Seth Cohen) doesn't like boats, so when he sails away in The O.C.: The Ties That Bind (2004) he is not in the boat at all. It is a 40-year-old stunt double in a wig that is actually sailing the boat.
When they were casting for the role of Marissa Cooper, writer Josh Schwartz had written to the producers that Marissa was "the most beautiful girl you had ever seen". They later said that they chose Mischa Barton because of her awkward beauty (meant in a good way), and as Barton herself later stated: "I was up against girls who looked liked your usual Californian beach blondes. Guess they decided to go against that."
Peter Gallagher and Mischa Barton were the first two actors to be signed to the show. The part that the producers had the most problems casting was Ryan, so the last actor to sign was 'Benjamin McKenzie' who got cast in the last minute.
When Ben McKenzie was cast as Ryan, the producers were amazed that he looked so much like a young 'Russell Crowe (I)'. Later in the show, Ryan takes his girlfriend Marissa to the theaters, and upon leaving she says that she didn't enjoy the film because "'Russell Crowe (I)' never did it for me".
Though being part of the "regular" cast, neither Jimmy Cooper (Tate Donovan) or Luke Ward (Chris Carmack) are in every episode, whereas Summer Roberts (Rachel Bilson), who was billed as a guest star for the first half of season one has been in every episode.
The entire cast (including newcomer Michael Cassidy, creator Josh Schwartz, and producer McG) was given the key to the city of Newport Beach, California by Mayor Tod Ridgeway on October 28, 2004 at The Historic Balboa Pavilion in Newport Beach.
When Tate Donovan signed on to play Jimmy Cooper in the pilot, he was so afraid that the show would never get picked up because he had been in so many pilots to shows that never materialized, that he thought that he was possibly jinxing them. He was therefore relieved when the show became a huge international success.
The character Kaitlin Cooper left the show after the first season to go away on boarding school. When she returns almost 18 months later in season three, Ryan Atwood says that he hasn't seen her since she was eleven. In the next episode the characters talk about Kaitlin leaving for boarding school when she was 12. A couple of episodes later Kaitlin celebrates her 15th birthday. 18 months after leaving for boarding school.
Melinda Clarke Julie Cooper-Nichol, actually read for the part of the perfect mother, Kirsten Cohen. In The Pilot, there just wasn't enough material for her to read as Julie, so she was brought in to read for the Kirsten part. The role of Kirsten eventually went to Canadian actress Kelly Rowan.
Due to labor union salary rules about filming outside of Los Angeles County, the show is actually filmed at Raleigh Studios in Manhattan Beach (40 miles away from the actual Newport Beach) to reduce filming costs.
Josh Schwartz, the creator of The O.C. (2003), who went to school in Providence, RI at the Wheeler School often includes references to his former teachers and friends, including on episode in which he refers to "Schmidt's bio", Schmidt having been his biology teacher. Renee Wheeler is also named after the Wheeler school.
Seth Cohen is watching TV and Ryan Atwood asks what he is watching. Seth replies with, "Sherman Oaks: The Real Valley" which is the reality version of the fake TV show, The Valley. There is a reality version of The O.C. called, Laguna Beach: The Real Orange County (2004).
Bret Harrison plays two separate characters; Danny in "The Rivals" and Swerve in "The Return of the Nana". Todd Sherry also plays two separate characters; a Waiter in the "The Pilot" and "The Best Chrismukkah Ever" and Todd in the "The End's Not Near, It's Here".
The trivia item below may give away important plot points.
To go out with a bang in the third season, the producers spread the word that one of five different leading characters would die in the last episode of the season. Rumors spread that they had actually shot all five deaths to avoid exposure of the truth about which character was dying. Whether or not all five death scenes were shot or not was never confirmed. However, Mischa Barton revealed it was her character that died on Access Hollywood an hour before the episode aired.