Actor Allusion: At Kaitlin's school, Marissa pretends to be an exchange student from London. Mischa Barton was born in London to British parents. Ooh, Me Accent's Slipping also applies and is lampshaded.
Casting Gag: Tate Donovan plays Jimmy Cooper (Father to Marissa & Kaitlin) and voiced the title character in Disney's Hercules. When the decision was made to finally introduce Frank Atwood (Ryan's father) in season 4, Kevin Sorbo got the role; Sorbo having played the title character in the unrelated Hercules: The Legendary Journeys.
Benjamin Mackenzie, Adam Brody, Rachel Bilson & Autumn Reeser were all in their early to mid twenties playing teenagers. Lampshaded in "The L.A."
Ryan (speaking of the star from the Show Within a Show): How does that guy play high school? Seth: It's Hollywood, man.
Averted with the Coopers. Mischa Barton was 17 playing 16 and Willa Holland was 14 playing 14.
Defictionalization: Before the series, "OC" was sometimes used in print as shorthand for "Orange County," but very rarely in speech, and never prefixed with a "the." Many Orange County residents scoffed at the show's title, but over the years, some of them really do refer to the region "the OC" without irony.
Executive Meddling - Resulted in the end of the Marissa/Alex lesbian relationship. Also, apparently a big factor for the quality decline in Season 3, as Fox wanted something 'promotable' to happen every week. The writers once got a note that said, "This is Fox, not Fox Searchlight".
Kaitlin was reintroduced in season 3 with a view to creating a spin-off following Kaitlin & her friends at boarding school. This was then retooled into keeping Kaitlin as a main cast member in the fourth season so they could keep the high school drama aspect of the show after the original group had graduated.
The writers tried to get Trey to return in Season 4 to finish his story arc.
In the same season, Al Gore was supposed to be Summer's GEORGE recruiter, but had scheduling differences.