'The OC' alum Rachel Bilson marks 15th anniversary of show with epic throwback picture

Actress Rachel Bilson played Summer Roberts on the hit drama "The OC." (Getty)

Actress Rachel Bilson is in shock the soap opera “The O.C.” is already 15 years old.

“15 year anniversary of The OC,” Bilson wrote on Sunday, while sharing a throwback photo of the crew. “I can’t believe it.”

Now 36, the actress continued, “I also can’t believe I wore a skirt I got at a thrift shop as a shirt.”

She added several hashtags, including one that was a hallmark to one of the famous lines from the show, which premiered on Aug. 5, 2003.

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The teen drama lasted four seasons on Fox, with the finale airing on February 22, 2007.

Bilson played Summer Roberts, a friend of Mischa Barton’s character, Marissa Cooper. She eventually married Adam Brody’s character, Seth Cohen, on the show.

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The photo she shared showed Barton, Brody and actress Melinda Clarke (Julie Cooper) to the right of her. Actor Peter Gallagher (Sandy Cohen) stood behind Bilson in the photo, while Kelly Rowan (Kirsten Cohen) and Ben McKenzie (Ryan Atwood) are on her left.