The O.C. had various shocking moments, and one that was a massive turning point for the series was Marissa Cooper’s (Mischa Barton) death, but this forced the series to improve. Although The O.C. focused on the ups and downs of Ryan (Ben McKenzie) and Seth's (Adam Brody) lives, the center of the story and its dramatic force was the relationship between Ryan and Marissa. Their toxic relationship led to some of the most memorable moments of the series, but their story together came to a definitive end in season 3

https://screenrant.com/oc-marissa-death-show-turning-point-better-improved/