Rachel Bilson has paid tribute to The O.C. on the show's 15 anniversary.

The actor, who played Summer Roberts in all four seasons of the hit show, celebrated the milestone by sharing a photo of the cast on Instagram.

She wrote in the caption: "15 year anniversary of The OC. I can't believe it.

"I also can't believe I wore a skirt I got at a thrift shop as a shirt #ifeelold."

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Just a few weeks ago Bilson opened up to People magazine about her time on the show which ran from 2003 to 2007.

"It was such a special time," she said. "The characters were really well thought out and there was so much chemistry between everyone. Even now, there are moments from the show that I look back on and I'm like, 'That was really cool!'"

Bilson also praised one of her former O.C. co-stars who has since gone on to become one of the biggest names in Hollywood.

"I laughed so hard with Chris Pratt," Bilson told the magazine.

"I remember laughing my a** off. He's seriously the funniest person I've ever come across."

Pratt starred in nine episodes of The O.C. playing a university activist called Che Cook.

To celebrate The O.C.'s 15th anniversary, here are some fun facts about the show.

Adam Brody initially blew it: The show's creator, Josh Schwartz, wasn't a fan of Adam Brody (who played Seth Cohen) when he auditioned for the show.

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"When he first came in to audition, it was pilot season and he was going on dozens of auditions, and he didn't really bother to learn the lines, so he just came in and I was like, 'What scene is he doing? Is this even from our show?' Schwartz said.

"We tried to find other actors, and our casting director, Patrick Rush, told me, 'I'm telling you, this Adam Brody is very special.' And I thought, 'That guy? I kind of hated that guy. He didn't even learn any of the words!' But he came back, learned the words, and he was great."

Shallow: Ever wonder why you never saw Seth or Ryan dive into the pool in the Cohen's backyard? It's because it was only 1.5m deep. The actors had to walk on their knees when they filmed a pool scene.

The other Marissa: The role of Marissa Cooper was almost played by Olivia Wilde instead of Mischa Barton.

"Olivia had just moved to LA and she was terrific, but she has such a strong persona that she didn't need rescuing," Josh Schwartz said.

Brody hates boats: In the show, Seth Cohen was an avid sailor but in real life, actor Adam Brody hated boats. According to Josh Schwartz, Brody agreed to film a scene on a boat for the pilot show, but it didn't go very well.

"He was green. He was just so miserable," Schwartz said.

After that experience, Brody made Schwarz promise that he'd never ask him to film a boat scene again. So when the script required Seth to get on a boat once more in a later season, they got a 45-year-old stunt double to wear a wig and film the scene instead of Brody.

Close call: Ben McKenzie, who played Ryan Atwood, was almost seriously injured during a fight scene when he fell through a glass window.

"Instead of fake tripping I actually tripped," McKenzie said. "The whole set just completely stopped. Thank god I was wearing this leather jacket. I didn't have a scratch on me."

Reunion storyline: Creator Josh Schwartz has revealed what he would like to happen if the cast ever get together for another season.

"I had actually pitched the idea that Ryan and Taylor were not together and that he had recently run into Alex Kelly and that they were actually starting to date. And Seth and Summer did not have babies yet because Summer was not going to have children until the world was a safer place and had reduced its carbon footprint."