Marissa Cooper is a fictional character on the FOX television series The O.C., portrayed by Mischa Barton. Marissa was among the original "core four" characters on The O.C. She is a privileged California native born into a wealthy family, residing next to the Cohen family's house. Throughout The O.C.'s storyline, Marissa is introduced to new characters who influence her perspective on life and her personality.

Marissa Cooper is characterized as the privileged yet troubled girl next door. Her designer-label-packed wardrobe masks her turbulent mood swings and hard-partying ways. Although Marissa is tall, thin and pretty, sadly, she is not much else. She is afforded all the privileges of a Newport Beach lifestyle, yet she fails to show any interest or engagement with the world around her. Like many of her peers, she is emotionally distant from her mother and becomes disillusioned with her mother's materialistic ways. As a result, she, like her sister Kaitlin, often rebelled against her mother. She is closer to her father, whom she stated was the "last thing keeping [her] sane" through the tumult of the first two seasons. Marissa was the social chair of The Harbor school. She organized the debutante ball rehearsals, school dances, carnivals, and other charity events. Simultaneously engaging in problematic romance and risky behavior, which in her case often go together.

Marissa's risky behavior was a factor in her on-off relationship with Ryan Atwood. For example, during Ryan's first Christmas in Newport Beach, he was furious to find her drunk due to his painful experiences as the child of alcoholic parents, but she did not understand his aversion to alcohol and thought that he was overreacting.

Marissa is introduced as a wealthy, beautiful socialite in Newport Beach, California, whose family (mother Julie, father Jimmy, and younger sister Kaitlin) go through dramatic changes over the seasons. She is a troubled person before the series begins, with a history of substance abuse and rebellious behavior. This is evident from a scene in the pilot where she is left on her front door by her friends, passed out from drinking. The backstory on her friendship with Summer Roberts showcases it as a powerful bond, forged during second grade. One of her most profound connections, the friendship is a central element on the show for three seasons. The show significantly focuses on Marissa's on-and-off again relationship with Ryan Atwood.

Viewers of the series have debated Marissa's sexuality. When she hooked up with Alex Kelly, her heterosexual identity became the source of speculation. Fans questioned whether Marissa was bisexual or just bi-curious. The question became one of the top queries that viewers wanted The O.C. creator Josh Schwartz to answer.

Schwartz insisted that the relationship between Marissa and Alex was not a rating stunt and that Marissa developed real feelings for Alex. Sarah Warn of AfterEllen.com said that despite Schwartz's statements, "Marissa's predominantly heterosexual past, the huge fan support for the Marissa and Ryan (Benjamin McKenzie) pairing, and the controversy The O.C. would court by making one of its principal characters bisexual, it's likely that her 'real feelings' for Alex will only last a few episodes." Within the series, Marissa refers casually to Alex as her ex-girlfriend.

Marissa's sexual orientation, other than her known heterosexual identity, was never confirmed by the series, and was reported to be "Marissa just experimenting with the fairer sex and not committing to a full-on romantic, sapphic relationship." The executives did, however, cite her relationship with Alex as an important one, and one that was successful with viewers.

Schwartz said that the network was "nervous" about the storyline and wanted it "wrapped up as fast as humanly possible and Alex moving on out of the OC."