In the pilot episode of The O.C., audiences were introduced to Bradley Stryker as Trey Atwood, Ryan's sonofabitch older brother who's basically the whole reason he got disowned and shipped off to Orange County in the first place—grand theft auto gets both of them arrested. After a year (essentially a season and change) in jail, Trey comes out a whole new man, literally. Instead of Stryker, Marshall-Green meddles his way into Ryan's life, and becomes the target in one the most parodied moments of the show's history: Marissa's epicly melodramatic, Imogen Heap-scored shooting.
It's unclear why producers traded Treys, though we'd assume it's because of acting capability. Marshall-Green recently starred in one of last year's most hyped about movies, Prometheus.