According to Kristin over at eonline.com
MISCHA BARTON IS LEAVING ‘THE O.C.’ THAT’S WHAT SOURCES CONNECTED TO THE SHOW TELL ME, THAT SHE MAY APPEAR AS A GUEST STAR NEXT SEASON, BUT WILL *NOT BE A SERIES REGULAR. A REP FOR FOX SAYS HE, QUOTE, “Can’t confirm or deny, but viewers will have to tune in to find out on May 18.” IN THE MEANTIME, I CAN TELL YOU WITH ABSOLUTE CERTAINTY THAT A MAJOR CHARACTER *IS BEING KILLED OFF IN THAT FINALE. IT COULD BE MARISSA, IT COULD BE ANOTHER ‘OC’ STAPLE. THIS IS A HUGE TWIST THE PRODUCERS HAVE BEEN PLOTTING FOR SIX MONTHS, AND *WHO IT IS HAS BEEN KEPT UNDER WRAPS BY SHOOTING MULTIPLE ENDINGS AND LEAVING CERTAIN PAGES *OUT OF THE SCRIPT. THIS DEATH WILL CHANGE EVERYTHING ABOUT ‘THE OC’ FOR NEXT SEASON.
Personally, anything that shakes up the OC can’t be a bad thing. This season has been generally awful, and it will take some major changes to get me excited about the show again. 
