Rachel Bilson -- Barbados Bikini Babe!

Ever since "The O.C." ('memba that) went off the air six years ago, Summer Roberts Rachel Bilson has really let herself go two pieces.

Having not aged since her days roaming the halls at Harbor High, the smokin' hot 31-year-old bathing beauty showed off her sunburnt rockin' bod in a sexy two-tone bikini on the beach in Barbados on Tuesday.

Rachel is giving us several gorgeous reasons why we should all start watching her latest TV show, "Hart of Dixie."