Harbor School's Senior Prom brings out the best and the worst in everyone. Taylor hits a high at prom, while Summer falls flat with her surprising prom date. Seth and Anna come up with a prom plan to win Summer back. Meanwhile, Marissa convinces Volchok to go to the prom and Ryan brings an unexpected old flame. When one of the girls is elected prom queen, it truly becomes a night to remember. Later, when Taylor's prom funds are missing, Ryan returns to his old ways and prom night turns brutal.

Taylor's ex-husband comes to Newport and causes a rift in her relationship with Ryan. Seth goes to Seattle to obtain Dr. Roberts blessing, while Summer "travels to the past" for 48 hours. Kaitlin finds love, while Julie finds problems with Kirsten.

Jimmy and Sandy run into a roadblock in their effort to re-open the Balboa Lighthouse restaurant. Summer and Seth take it upon themselves to finally make Marissa feel more at home at Jimmy's. Everyone is shocked when Caleb makes a proposal, which sparks a near-fatal accident for Luke. Marissa, and Sandy and Jimmy each strike deals with Caleb. An old O.C. friend moves away.

Jimmy must make once more the difficult decision to leave or stay with his family; Kirsten returns to the Cohen household while Charlotte has to leave her house; Marissa and Ryan face a possible separation; Taylor lords over the school musical and makes Summer and Seth her stooges.

Seth and Kirsten want to avoid facing the world after their respective displays at Thanksgiving. Sandy has some advice for Jimmy, which doesn't sit well with Julie. Ryan and Luke find themselves getting along after an O.C. loved-one comes out of the closet to face negative reactions.

Kaitlin takes down the school mean girl by throwing a rager at her house. Sandy and Kirsten find time to bond with Ryan while Seth is away visiting Summer at Brown but all he finds is an empty dorm room. Meanwhile Julie continues work on the dating service and finds out a new surprise.

Chrismukkah once again returns to the Cohens. Ryan and Taylor have an accident while putting up the holiday lights and they end up in a sort of parallel universe, where all is not like it is meant to be, since Ryan does not exist. While - in the real world - everybody is worrying about them, except Taylor's mom, they eventually manage to fix things, also thanks to a letter from the past.

Ryan overcomes his indecision and competes with Henri-Michel for Taylor's affections. Che leads Seth on a trek of spiritual renewal. Meanwhile, Kirsten and Julie have to do some damage control.

It's time for the annual Cotillion ball. Cherished by many, hated by most, it's the time when Newport's finest young ladies enter society. More information about Jimmy's business trouble comes to light, and Kirsten's loan to Jimmy is not happy news for Sandy.

Sandy and Jimmy have plans to sign papers for the Lighthouse... but is Jimmy ready? Oliver makes his plans official to Ryan, but Ryan just cannot catch a break from anyone and finds himself in an all too familiar position. Summer has a "big" new boyfriend... for a little while. And Julie gets a job.

When an old friend returns to Newport, Ryan decides to help her, which causes problems for him and Sadie. When Summer and Seth run into another rough patch Taylor tries to help them out. Sandy has to deal with more problems with the Newport Group and Marissa must deal with being alone.

Julie and Dr. Roberts announce their engagement and the Cooper ladies move in with Summer and her dad. Meanwhile, Marissa drifts away from the group and Summer when she seeks solace in the bad company of Volchok and his crew. Ryan tries to move on with Sadie, but Seth's concerns for Marissa get in the way. At the Newport Group, Sandy's woes with Matt continue and Kirsten surprises Seth with a night out.

Kirsten's father, Caleb Nichol, comes for a visit along with his new girlfriend, Gabrielle. 24-year-old Grandma Nichol has arrived. Kirsten finds her job in jeopardy, while Sandy considers making a change. Much to Ryan's disappointment, Luke decides he wants a second chance with Marissa... but who will Marissa choose? Summer makes the moves on Seth, who wonders if her feelings are real.

Sandy, Ryan and Seth prepare for Kirsten's upcoming 40th birthday party, however Kirsten learns some news that will change her life forever. Kaitlin tries to stop her mother having a secret love affair. Taylor visits a therapist and Ryan pretends that he doesnt miss her. Elsewhere Che finally learns that Seth isn't his soulmate.

Seth and Summer come upon a fortune teller whose prediction makes the two question whether they are truly meant for each other. Ryan gets back in touch with his father (with a little help from Taylor). When the two teens discover a love connection between Frank and Julie despite her pending engagement, a Valentine's competition is sparked between "Team Bullit" and "Team Frank". Meanwhile, with a new baby on the way, Kirsten finds herself dwelling on an old secret involving Jimmy Cooper.

The Cohen family says goodbye in the series finale as the episode jumps six months after the earthquake to show the Cohens, Ryan, Taylor, Summer and the Cooper women trying to piece their lives back together. The Cohens find out that fixing up their old mansion would cost more than rebuilding it but Ryan and Seth see this as an opportunity to give Sandy and an expecting Kirsten a surprise that'll change their lives.

Kaitlin prepares herself to head back to school. Ryan has doubts about his relationship with Marissa after helping out Sadie. Kirsten and Julie make plans for a Valentine's Day party.

Ryan thinks about not inviting Marissa to his 18th birthday after their falling out but Sandy tries to get him to invite a relative. After months of avoiding telling Summer and Marissa the truth Neil and Julie tell Summer and Marissa the truth.

When college acceptance and rejection letters are received the kids must consider what path they are going to follow. Ryan and Sadie are trying to decide what they will do after high school. Taylor holds a college party and Summer and Marissa fix their friendship problems.

Ryan and Taylor may be heading out on a Las Vegas road trip for New Year's Eve, but there's even more fun in store when Seth and Summer join them. Meanwhile Julie doesn't trust NewMatch's new accountant, and Sandy suspects the reason why.

Hailey discovers she isn't as welcome as she might have thought. Perhaps the Cohen house is full? Luke makes some new friends, while Seth and Anna try to tell a friend about their decision. Rooney sings, a lot, and Oliver noses, a lot. Trouble brews.

When Summer and Marissa run into Grady Bridges, the star of Summer's favourite prime-time soap 'The Valley', he invites them to an LA party, and they accept. On the road trip to LA, Seth, Summer, Marissa, and Ryan run into Hailey, now working at a strip club. Jimmy and Sandy present their restaurant to Newport's finest and approach Caleb for some financial help.

Marissa plots to ruin her mother's happiness by inviting someone from her mother's past to the wedding shower, while Sandy attempts to hide Theresa's return to the O.C. from Ryan. When Ryan discovers Sandy's lies and Theresa's reasons for coming back, he puts himself in a dangerous situation. In the meantime, Seth's unimpressive first meeting with Summer's father leaves their relationship in doubt.

As Sandy continues working on Rebecca's case, Kirsten questions how strong their feelings are for one another. Seth continues his downward spiral in the aftermath of Zach and Summer's Valentine's night together, while Marissa confronts her fear about her relationship with Alex. Caleb offers Lindsay a place in his family, but not without a paternity test.

When Ryan and Sadie go on a road trip to search for one of Johnny's relatives they end up in Ryan's old neighborhood in Chino, which brings the two closer together. While the two are away Marissa is implicated in Johnny's death and Seth and Summer learn the truth about Julie and Dr. Roberts new secret.