On the 5 August it will be 20 years since Noughties teen drama The OC first hit screens. The show, which followed the messy, glamorous lives of teenagers Marissa, Ryan, Seth and Summer, living in the affluent California surf town Newport Beach, was an immediate hit when it was released in 2003. The fashion was an integral aspect of why the show such a hit and was as important as the indie soundtrack in bringing this world to life.

The “Newpsies” wore Juicy Couture velour hoodies to travel from their McMansions to “yogalates”, Marissa Cooper was the teenager who wore Chanel to prom and Ryan Atwood and his white vest tops moved into the Cohen’s pool house. An honourable mention also goes to Seth Cohen for his band T-shirts and Chrismukkah jumper. From ruffled miniskirts to Uggs to super low-rise jeans; the programme was an originator and escalator of some of biggest trends of the mid-Noughties.

Costume designer Alexandra Welker is to thank for that. “When it came out, ‘casual California style’ was not a thing in culture,” says the stylist, who was inspired by ’70s and ’80s skate and surf culture. “But it very much became a thing.” In fact, while no designer would lend Welker clothes for a charity fashion show in the pilot – “we had to borrow things from Rent The Runway” – by the time season one got going? “Brands were throwing samples at us,” she laughs, explaining that they often turned them down. “There’s a Chanel dress that Marissa wears when she drunkenly crashes a car. I think we filmed her climbing out of a window in that.”

With The OC’s big anniversary imminent – and Y2K style very much back in the mainstream once again – Vogue caught up with Welker to get her tips for doing 2003-dressing the “casual California” way.

Know your Noughties California brands