American teen drama The OC first aired in 2003 and it starred actors including Guardians Of The Galaxy’s Chris Pratt and Divergent actress Shailene Woodley.
The show follows a group of friends who live in the upper-class neighbourhood of Newport Beach, Orange County, California.
In The OC, McKenzie, 44, plays Ryan Atwood, a teenager who is taken in and adopted by the Cohen family.
Remembering the show, McKenzie posted a photo of himself to Instagram with actor Adam Brody, who played Seth Cohen.
The post said: “The OC premiered 20 years ago today.
“I keep showing my kids pics to prove how cool I used to be, but for some reason they don’t seem to get it.”
Bilson, 41, also posted a photo to her Instagram with the simple caption “Twenty”.
The actress played Summer Roberts, who had a love interest in Brody’s character Cohen.
Director of Don’t Worry Darling Wilde, 39, also reminisced on the show and reposted a photo of herself with Mischa Barton to her Instagram story.
Wilde said: “Ok I see it’s (The OC) having a moment because it premiered 20 years ago.”
She added “Mischa = One of the most beautiful faces I’ve ever seen. I remember just being like whoooaaaaa.”
Wilde had played Alex Kelly, who dates Cohen and Barton’s character Marissa Cooper.
Melinda Clarke, who starred as Julie Cooper in the show, posted a photo of herself and Peter Gallagher (Sandy Cohen) to her Instagram, and could not seem to believe how old the show was.
She said: “Wait Whut?! We’re how old? #20yearstoday #foreveryoung.”
Josh Schwartz, creator and executive producer of The OC, reposted a photo from the show to his social media which said: “Yes I am feeling old. Thanks.”