Marissa Cooper, played by Mischa Barton, was killed in a car crash.
Barton's character was central to the show, which follows four wealthy Newport Beach teens through the emotional ups and downs of high-school life.
Barton was already a familiar face when she began starring on "The O.C." in 2003. She worked as a model and appeared in several films, including "The Sixth Sense," and TV's "Once and Again." She is also a spokesmodel for Neutrogena and the collection bebe clothing line.
Rumors flew in recent months that she was ready to leave "The O.C."
Earlier this week, Barton told "Access Hollywood," "My character has been through so, so much and there's really nothing more left for her to do."
Barton, 20, has a film arriving in 2007, "Guilty Pleasures," and other projects in the works.
Marissa's death came at the end of an otherwise upbeat episode.
The class of 2006, which includes Marissa, Ryan, Seth and Summer, graduated from Harbor High School. Seth and Summer were accepted by the same college. Ryan was reunited with his mother, who gave him a Land Cruiser as a graduation gift. Marissa's mom gave her a pearl necklace, while her dad gave her an opportunity to work with him on a yacht set to sail around the Greek islands.
Ryan was taking Marissa to the airport in his new SUV when they were antagonized at high speed by a drunken and vengeful Volchok, who they both had a history with. Volchok swigged from a flask as he chased the pair in his car.
Ryan lost control of the Land Cruiser and crashed, the vehicle rolling off the road. He pulled Marissa from the wreckage just before the vehicle caught fire.
There, on the side of the road, she died in his arms.