Welcome to Orange County, an idyllic paradise home to California's wealthy individuals. Here, it seems that everything and everyone is perfect, but behind closed doors is a world hidden loyalties, identities, and lives. When Ryan Atwood, a tough, yet intelligent 16-year-old, is arrested for helping his brother steal a car, pro-bono public defender Sandy Cohen is assigned his case. But when Ryan is abandoned by his mother, Dawn, Sandy, seeing himself in Ryan, becomes a bigger part of his life than either ever expected. Sandy, much to the chagrin of his wife, Kirsten, Sandy invites Ryan to stay with them until his situation is worked out. However, Kirsten worries that Ryan will be a bad influence on their teen-aged son, Seth. Meanwhile, Ryan becomes attracted to Marissa Cooper, the girl next door who seems to have an effortless and perfect life. But neither of them are aware that the indiscretions of Marissa's father, Jimmy, make shatter the perfect world she's living in. Later, Ryan and Seth find common ground after a fight Newport teenagers, including Marissa's boyfriend, Luke.

Thinking that he'll be put into foster care, Ryan attempts runs away from the Cohens, but is stopped when Seth figures out his plan. However, Seth doesn't want to lose his only friend, so he hatches a plan to keep Ryan in Newport. Seth, with the help of Marissa, hides Ryan in an unfinished home in Kirsten's newest housing development. However, Luke and his friends discover the plan, resulting in a bloody brawl between Ryan and Luke. Meanwhile, Sandy and Kirsten find Ryan missing, resulting in more arguments about Sandy having Ryan stay with them.

Sandy once again represents Ryan, but this time for the arrest following the model home fire. After his release, Ryan searches for his mother, and once she's found, Ryan agrees to give her a second chance. However, when Dawn accompanies the Cohens and Ryan to a Casino Night, Dawn drinks heavily and Kirsten sees how unfit Dawn is in being a mother to Ryan. Meanwhile, both Sandy and Julie find out that Kirsten lent Jimmy 0,000 to help him with his debt, resulting in an argument between Sandy and Julie.

After the events of Casino Night, Kirsten and Sandy decide to become Ryan's legal guardian and Ryan officially becomes a part of the Cohen family. For the upcoming Cotillion, Ryan is paired up with Anna, and Summer, much to her dismay, is paired up with Seth. After rehearsal, the kids all go to a pre-Cotillion beach party, but much like the first beach party, Seth and Ryan are thrown into an unwelcoming crowd. This time, however, Ryan is able to resist fighting Luke. Meanwhile, Sandy learns that Jimmy's 0,000 loan from Kirsten has put her into Jimmy's fraud investigation, and the community's anger towards Jimmy's fraud hits its end during Cotillion.

Ryan, wanting to not be as dependent on the Cohens, gets a job at the Crab Shack. There he meets Donnie, and due to their similar backgrounds, they strike up a friendship. But worlds collide when Donnie invites Seth and Ryan to a party. Later, Ryan finally gets the courage to ask out Marissa and Sandy offers Jimmy legal help.

Kristen nervously prepares for her father, Caleb's, birthday party, and is thrown off when she meets his new girlfriend, Gabrielle. After constant criticism from Caleb about Ryan, the Newport Group, and his party, Sandy suggests to Kirsten that they move away from this life and return to the life they had before moving to Newport. Later, Caleb turns Jimmy down for a job, but becomes increasingly interested in Julie. Meanwhile, Seth's years of dreaming finally become a reality when he and Summer share their first kiss, and Ryan has to fend off Gabrielle's advances.

With summer break drawing to a close, Seth is able to convince Ryan to plan a trip to Tijuana, and Summer convinces Marissa to come along as well. But the trip runs into disaster when Seth drives off the road, which damages the vehicle. While the car is repaired, the foursome has no choice but to stay in a grungy motel. When they finally make it to Tijuana, Marissa is devastated to find Luke with another girl, but Ryan is there to comfort her. Back in Newport, Jimmy and Julie finalize their divorce and Jimmy moves out. While helping Jimmy paint his new apartment, Kirsten's old feelings for Jimmy resurface.

With Marissa in the hospital after her overdose, Julie takes her anger out and puts the blame on Ryan. Despite threats from Julie and the risk of losing his spot at Harbor, Ryan does all he can to make sure Marissa isn't sent to San Diego for treatment. Meanwhile, Sandy adjusts to his new job and his friendliness towards Rachel, a co-worker, raises Kirsten's suspicions.

During the first day of school, Summer continues to ignore Seth while in public, but privately she's jealous of his budding relationship with Anna. Kirsten also feels jealous when Sandy spends long hours at the office with Rachel, and things are made worse when Sandy is assigned to a case against Caleb. Luke continues to try to get Marissa back, but his attempts make Ryan jealous, resulting in a fistfight between the two. Later, Ryan tries to make up for the fight when he gets Marissa alone with him on a Ferris wheel.

Now that Marissa and Ryan are officially a couple, they decide to take things slowly. When Seth and Anna begin to get closer, Summer has a hard time hiding her true feelings for Seth. Tensions between Sandy and Kirsten continue to grow as Sandy continues to work on the case against Caleb. Trying to patch things up with Marissa, Julie invites Marissa, Ryan and Jimmy to her charity event. But their fragile relationship falls apart when a surprise couple is revealed.

It's Thanksgiving in the OC, and Seth is forced between Summer and Anna, but the process lands Seth in hot water. Meanwhile, Ryan, accompanied by Marissa, travel to Chino to visit Trey, and their quick trip infuriates Julie. When they return, the mounting tensions explode, making it an OC-style Thanksgiving to remember.

In the aftermath of Thanksgiving, Anna and Summer are still angry at the way Seth treated both of them and Seth continues to try to make things right. Caleb and Julie's brief relationship is ended when Julie wants a commitment, but Caleb isn't ready to do so. Meanwhile, Ryan and Luke settle their differences when a secret is exposed, and Sandy learns what Kirsten is hiding from him when he meets with Jimmy to work through is legal troubles.

Seth, staying true to his unique self, created his own holiday by combining Hanukkah and Christmas into Chrismukkah. And this Chrismukkah will be one to remember as Anna and Summer force Seth to choose between them. Ryan is reluctant to celebrate due family problems during past holidays, but ends up helping Marissa, who as turned to alcohol, cope with her own family issues. Meanwhile, Kirsten tries to take her first vacation in years while Sandy continues to work on the case against Caleb's company.

Things are bound to go wrong when Kirsten's rebellious sister, Hailey, returns to Newport. Ryan and Marissa decide to spend New Year's watching movies together, but when Marissa is invited to Oliver's party, she decides to go to the party instead. Hailey gets Kirsten and Sandy to go to a party, which ends up to much more than they bargained for, and in their absence, Hailey throws a raging party for her friends. As the clock ticks down, Ryan rushes across town to be with Marissa when the clock strikes midnight.

Hailey moves in with the Cohens, and proves to be a difficult house guest. Kirsten is unable to confront Hailey about her problematic tendencies, so Sandy takes things into his own hands. Meanwhile, Seth and Anna have a hard time telling Summer about their new relationship and Ryan becomes increasingly jealous about Marissa spending all of her time with Oliver. But when Oliver is arrested at a concert for trying to buy drugs, Ryan is able to enlist the legal help of Sandy.

Ryan, Seth, Marissa, Anna, Luke and Summer are invited to spend a weekend with Oliver at his parent's luxury home in Palm Springs. Oliver quickly gets on Ryan's nerves when he begins to flirt with Marissa and making sure that Marissa and Ryan get no time to be alone together. Summer does all she can to annoy and come between Seth and Anna, but refuses to admit her feelings for Seth. The weekend takes a turn for the worst when Oliver leaves and later calls Marissa to tell her he's taken a handful of pills. Back in Newport, Kirsten talks to her father about Hailey's problems and Sandy and Jimmy decide to go into business together.

Sandy and Jimmy take the first step in going into business by buying the Lighthouse restaurant. Kirsten has a tough time at work when Julie is hired as a designer for the Newport Group. Despite being with Anna, Seth becomes jealous when he sees Summer and her boyfriend together. Meanwhile, Ryan risks his enrollment at Harbor to expose the truth about Oliver.

Caleb drops responsibility of the company in Kirsten's lap and forces her to fire Julie. Kirsten also faces stress at home when she and Sandy begin to question if they did the right thing by bringing Ryan into their household. Meanwhile, Ryan and Luke team up to bring Oliver down, resulting in a potentially deadly showdown when the truth is revealed to Marissa.

Marissa tries to get her relationship with Ryan back on track, but Ryan is reluctant to forgive her about the situations surrounding Oliver. Things become harder for Marissa when Theresa, Ryan's ex-girlfriend from Chino, comes to Newport to cater the Valentine's Day dance. Meanwhile, Seth and Summer take their friendship to a new level and Jimmy learns that he has a secret admirer living in the Cohen house.

Ryan and Marissa decide to start over as friends, but it proves to be a hard task when Ryan begins to spend more time with Theresa. Privately, Seth and Summer's relationship is going good, but in public, Summer hides the fact that she's with Seth. Seeing this, Seth decides he has to teach Summer a lesson. Meanwhile, Jimmy begins to be suspicious of Julie and Luke; Ryan learns why Theresa is in Newport; and Sandy finds himself involved in another mess Caleb created.

After Theresa decides to stay and Newport, Ryan helps her find a job and a place to stay. Julie and Luke's affair is almost exposed when Jimmy sees them together. Meanwhile, Anna decides to move back to Pittsburgh and Seth convinces himself that she's leaving because of him, causing him to rush to the airport in order to stop her.

Marissa and Summer decide to take a trip to Los Angeles, and Seth, worried Summer will have fun without him, invites himself to go with. Ryan decides to tag along as well in order to make sure that Marissa doesn't find out about her mother's affair with Luke. Meanwhile, Sandy and Jimmy find themselves in a bind.

When Sandy finds out that his mother is coming for a visit, the Cohens have to prepare for a Passover dinner with 'the Nana'. Summer, wanting to get on the Nana's good side, decides to do some preparing as well. But when the Nana arrives, her news shocks the Cohens. Meanwhile, when Marissa goes missing, Ryan heads out to track her down, and finds her hiding in the last place he expected.

After finding out about their affair, Marissa refuses to hear any explanation from either her mother or Luke. Luke, feeling depressed, drinks too much and gets into a near fatal accident. Only then does Ryan and Marissa agree to listen to Luke. Meanwhile, prior to the opening of The Lighthouse, Jimmy and Sandy get learn that they were denied their liquor license. Now, Sandy must decide between getting the license or keeping Jimmy as a partner. Later, Caleb uses the restaurant opening to ask Julie an important and surprising question.

Now that Julie's engaged to Caleb, a reluctant Marissa and Kirsten throw her a bridal shower. Theresa returns to Newport and the reason for her return turns Ryan's life upside down. Meanwhile, Seth meets Summer's father, but the meeting is a disaster for both of them.

Seth, Ryan, Sandy, and Caleb head to Las Vegas for Caleb's bachelor party. Meanwhile, back in Newport, Kirsten throws together a bachelorette party for Julie. Marissa and Theresa begin spending time together, and Marissa learns more about Theresa than she bargained for.

Julie and Caleb's wedding day arrives, and becomes a life-changing day for more than the newlyweds. As the vows are said and the couple is pronounced husband and wife, events unfold that change the lives of several people forever.