After his stint on Everwood, Pratt brought the funny back to The O.C. in season four as Che, Summer's activist college friend.
"He's hilarious and he brought so much life to the set and the cast," creator Josh Schwartz gushed of the Guardians of the Galaxy star. "And you start to feel all the other actors have to raise their game 'cause they knew this guy was walking in and he was really, really fun.
Long before he played Schmidt on Fox's New Girl, Greenfield played a young Sandy Cohen in a flashback to his college days at Berkeley.
Though Willa Holland eventually took over the role, the Big Little Lies star originated the role of Kaitlin Cooper, Marissa's horse-loving and troublemaking little sister. China may have alopecia but the former Mini-Coop has an Emmy nom!
After losing out on the role of Marissa to Mischa Barton, producers brought Wilde on in season two as Marissa's love interest, Alex, who managed the gang's local hotspot, The Bait Shop. Who could forget how much Fox hyped the kiss you didn't want to miss?
The Vampire Diaries star made a memorable two-episode stint as Donnie, the guy who shot Luke (aka "Abercrombie and B--ch"), in season one. He and star Ben McKenzie bonded on-screen and off, remaining friends to this day.
Before he broke hearts as Denny on Grey's Anatomy and broke heads as Negan on The Walking Dead, JDM made a brief appearance in season two when he played Joe Zukowski, an inmate with ties to Sandy's former college girlfriend.
The former Disney Channel star played a young version of Taylor Townsend in a season four flashback.
While he went on to become Rachel Bilson's main love interest on The CW's Hart of Dixie years later, he actually shared the infamous kissing booth duties with her way back in season one. But his run on Brad on the show didn't end on the best of terms.
"I actually got fired from The O.C." Wilson once revealed to E! News. "I actually hoped that Josh Schwartz didn't remember me as the being the guy who on my last day of work showed up to the set six hours late, and the result of which was promptly canned."
Before they fell for each other while starring on Gotham together, Baccarin appeared on her husband's show for three episodes in season three as Maya Griffin, Sandy's boss' daughter.
In the same episode as Hanks, Paris Hilton played a version of herself when the gang visited L.A. In the episode, Paris revealed to Seth that she's writing her grad school thesis on magical realism in American fiction. Oh, and she kissed him. That's hot.
The singer guest-starred in the final season as Will Tutt, a band geek who brefriends and flirts with Kaitlin Cooper.
The Aquaman star played a nameless-but-slightly-b--chy salesgirl in the season two episode in which the Core Four gets stuck in a mall overnight.
The Pretty Little Liars star played Kaitlin's boarding school roomie in season three.
Years before they played frenemies on Pretty Little Liars, the pair actually played BFFs Leah and Riley in a season four episode.
The Star Wars mastermind himself made Seth Cohen's dreams come true when he appeared as himself in season two when he expressed interest in turning Seth and Zach's comic book, Atomic County, in a movie. Seth ended up leaving the meeting to meet Summer at their prom.
"That was sort of like, [absurdly] 'What if we tried to get George Lucas to appear on the show?' It seemed like a real impossible task. But he had a teenage daughter who was a fan of the show," Josh Schwartz said. "I still have the Storm Trooper helmet that I got as a thank-you gift sitting in my office. That was a huge honor. And I got to go to Skywalker Ranch to a screening of Revenge of the Sith. I got to bring my dad, which was a long overdue thank-you to him for taking me to see Return of the Jedi when I was a little kid."
Ryan's absentee dad who finally showed up in season four and romanced Julie Cooper was played by none other than Hercules.
The future 90210 star made a brief but memorable appearance in season three when she was the girl whom Marissa found Volchok hooking up with...at her prom. Her name om IMDB? "Hot Girl." Hey, it could be worse!
Before he tortured Bella in Twilight, Gigandet was torturing Marissa Cooper as her troubled season three BF-turned-killer Volchok.
The future Mrs. Ian Somerhalder and Twilight star played Sadie, Ryan's season three love interest after breaking out in a big way when she starred in and wrote the 2003 hit film, Thirteen.
Another Twilight alum, Rathbone played Justin, Kaitin's schoolmate from boarding school who accused her of stealing. How rude.
Years before she was stirring up trouble for Rachel Bilson on Hart of Dixie, she stirred up trouble for Seth and Summer when she made a brief appearance as Mary Sue, a girl Seth meets on Spring Break. Summer is not pleased when she sees on TV that he entered a competition in which he has to lick whipped cream off of Mary Sue's body.
The rapper kicked off his acting career with an appearance as himself during a pool party in season two, also performing "Bring 'Em Out."
In a super-meta moment, Hanks appeared as Grady Bridges, the star of The Valley, the hit-teen-drama-within-the-hit-teen-drama.
The Ugly Betty star was hated by viewers when he played Dean Hess, who was having an affair with Taylor Townsend.