Ryan Atwood is a troubled, 16-year-old kid living in the rough, crime-ridden neighborhood of Chino located in San Bernadino, California, who gets thrown out of his own house by his alcoholic and neglectful mother and her abusive, alcoholic boyfriend after Ryan gets into trouble with the law when his self-destructive older brother, Trey, implicates him in the theft of a car. Ryan is appointed, Sanford 'Sandy' Cohen, a public defender, who takes the homeless youth to his house in the posh and picturesque neighborhood of Newport Beach, Orange County, (the OC) and lets Ryan stay in the guest house against the protests from Sandy's wife Kristin and their bumbling and socially handicapped son Seth. Ryan forms a bond with Marissa Cooper, a friendly, but similarly emotionally troubled teenage girl living next door who introduces Ryan to the world of beach parties and life of the idle rich in Newport Beach. But Ryan's feelings for Marissia are hampered by Marissa's egotistical jock boyfriend, Luke Ward, whom takes an instant dislike to Ryan. Marissa's father Jimmy, a financial stockbroker, begins having problems with his business and tries to get out of the hole he's in to keep his narcissistic, gold-digging wife, Julie, from finding out.

When his mother disappears and faced with the prospect of life in a group home, Ryan decides to run away from Newport, but Seth and Marissa convince him to stay and hide in a vacant building project. Meanwhile, Jimmy discusses his recent financial woes with Kirsten, and she offers a solution. After the jealous Luke witnesses a close encounter between Marissa and Ryan, he confronts Ryan and a fight breaks loose in the house where their quarrel accidentally starts a fire. Ryan nearly dies in the flames, until surprisingly Luke returns and saves Ryan's life, which unfortunately reveals Ryan's continuing presence in Newport leading to both him and Luke getting arrested for arson.

Ryan has a difficult stay in a Juvenile Hall prison for the arson charge. Seth makes Kirsten aware of his anger over Ryan's situation, and Kirsten makes a compromise to bail Ryan out of jail to stay with them, temporary. Later, Ryan's mother, Dawn, makes a surprise visit to Newport, and things don't go quite as she and Ryan may have planned due to her constant drinking. Meanwhile, the monetary secret involving Julie, Jimmy, Kirsten and Sandy comes to a head when Sandy finds out of Jimmy's financial problems about the money investments he lost and unwisely 'borrowed' without permission from his clients bank accounts to cover it up and lost that money too, and of Kristen's lending their money to help Jimmy which brings out more jealousy in Sandy for he's aware of Kristen and Jimmy's romantic past.

During rehearsals for Newport Beach's annual Cotillion Ball, Ryan continues to pursue Marissa and asks her to be his dancing partner, while a continuing jealous Luke tries to break them up. Seth as eyes for the new girl in town, Anna Stern, but becomes torn over his feelings for Summer Roberts, the outgoing socialite he's had a crush on for years. Meanwhile, Sandy becomes upset over Kristen's loan to Jimmy for his financial problems which finally come to light at the ball when one business client decks him in the face after learning that Jimmy lost all of their money.

Ryan takes a job at the Crab Shack diner where a co-worker, named Donnie, might mean bad news for the Newport kids when he takes Ryan and Seth out to party queen Holly Fischer's parties where drugs and alcohol flow freely. Ryan asks Marissa out on a date, and when Luke again tries to intervene, Donnie decides to take matters in his own hands when he tries to starve off Luke... with a gun. Meanwhile, Sandy attempts to help Jimmy sort out his life, and gives him some bad news that be may go to jail for his unauthorized loans to cover his tracks. Also, Julie reveals a long-held secret to Kirsten about her past.

Kirsten's uptight father, Caleb Nichol, comes for a visit along with his new girlfriend, Gabrielle, a 24-year-old model. Caleb does not hesitate to speak his mind and immediately angers both Sandy and Ryan, and browbeats Seth, with constant criticism with their backgrounds, while Gabrielle makes a move on Ryan. Kirsten finds her job at Caleb's company, Newport Group, in jeopardy, while Sandy considers making a change. Meanwhile, Luke approaches Marissa and tells her that he wants a second chance with her, much to Ryan's disappointment. Also, Summer makes the moves on Seth, who wonders if her feelings are real or if she just using him.

Ryan, Seth, Marissa and Summer plan an end-of-summer trip down to Tijuana, Mexico (aka: the TJ). Luke wants to tag along with them to try to get back together with Marissa, but she later changes her mind about him when she catches Luke making the moves on Holly Fischer. Back in Newport, Jimmy has plans of his own to get out of his financial and legal problems, which don't sit well with Marissa or Julie. Sandy gets a new job at a large prestigious law firm, which Kirsten can't understand why.

While Marissa is in a hospital in Newport recovering from her near-fatal drug overdose in Mexico, Julie unfairly blames Ryan for her condition and forbids him from seeing Marissa, who does not appreciate the help. Meanwhile, Sandy starts his new job, but finds himself surprised by the case work. Kirsten finds herself surprised by Sandy's new co-worker, the attractive Rachel. Also, Ryan enrolls at his new school, with conditions after his past life in Chino is revealed. Ryan and Sandy then stage an intervention for Julie about her handling of everything in which she lets Marissa stay with Jimmy who moves out and rents a small apartment for himself.

At the opening day of the new school year, Ryan doesn't feel like he fits in at his new school of Harbor High, and Luke doesn't make the situation any better when he and his jock friends continue to bully and harass both him and Seth, who does like being at school any more than Ryan. Meanwhile, Sandy accepts a case at his new job, a case that will test his relationship with Kirsten and her father Caleb. Anna and Seth hatch a plan to make Seth feel better with his current situation. Also, Ryan and Luke confront Marissa and force her to choose one of them.

Having chose Ryan over Luke, Marissa wants to make plans for their first romantic date together, while Ryan does not know how to behave for his first date with Marissa. Seth takes matters into his own hands involving getting to like Summer, but gets taken in with Anna whom also develops feelings for him too. Meanwhile, Kristen finds Caleb with another woman, that being Julie Cooper, while Sandy continues to resist temptation with his co-worker Rachel.