Started - Season 1 Episode 1: Pilot
Ended - Season 1 Episode 4: The Debut
Why: Luke got jealous of Ryan and Marissa hanging out together too much.
Started - Season 1 Episode 6: The Girlfriend
Ended - Season 1 Episode 7: The Escape
Why: Marissa caught Luke cheating on her with one of her best friends in Tijuana.
Started - Season 1 Episode 9: The Heights
Ended - Season 1 Episode 17: The Rivals
Why: Ryan attempts to inform Marissa of Oliver's lies and his unhealthy obsession with her, though she refuses to believe him. After repeated attempts to get her to turn against Oliver, Marissa breaks things off with Ryan.
Started - Season 1 Episode 23: The Nana
Ended - Season 1 Episode 27: The Ties That Bind
Why: After discovering that Theresa is seemingly pregnant with his child, as the two had been in a relationship during Ryan and Marissa's break-up, Ryan decides to move back to Chino with Theresa despite his feelings for Marissa.
Started - Season 2 Episode 20: The O.C. Confidential
Ended - Season 3 Episode 16: The Road Warrior
Why: With Johnny just dying and the new girl Sadie coming along it's made things complicated. Ryan starts to think that Marissa and him are not really working out so he calls her and she agrees with him.
Started - Season 3 Episode 24: The Man of the Year
Ended - Season 3 Episode 25: The Graduates
Why: Marissa dies in an automobile accident caused by a drunken Volchok.
Started - Season 2 Episode 1: The Distance
Ended - Season 2 Episode 8: The Power of Love
Why: DJ realized that Marissa was only dating him to get back at her mom.
Started - Season 2 Episode 12: The Lonely Hearts Club
Ended - Season 2 Episode 16: The Blaze of Glory
Why: Alex realized Marissa still had feelings for Ryan and wasn't that into her like she was to her so Alex broke it off with her.
Started - Season 3 Episode 18: The Undertow
Ended - Season 3 Episode 23: The Party Favor
Why: Marissa catches Kevin making out with another girl at prom the slaps him across the face.