O is of Korean and Polish heritage. She was born in Busan, South Korea, the daughter of a Korean mother and a Polish father. The family eventually moved to Englewood, New Jersey, where she grew up and graduated from Dwight-Englewood School. About her childhood, she stated that "it's almost embarrassing how well-behaved I was, which is probably why I do things like spit water on myself on stage as an adult". She attended Oberlin College before transferring to New York University's Tisch School of the Arts.
O is best known as the lead vocalist for the band Yeah Yeah Yeahs. She has also been noted for her sense of fashion, wearing ostentatious outfits made by her friend, fashion designer Christian Joy. In the early days of the band, she became well known for her outrageous antics during live shows. Their first ever gig was opening for The White Stripes.
During a tour for the 2003 Livid Festival in Australia, at a sideshow at The Metro in Sydney, O accidentally danced off the stage and was reluctantly taken to a hospital. A few days later, at the Sydney leg of the Livid Festival, she appeared in a wheelchair pushed onstage by Angus Andrew.
In 2007, O was placed at number three on Spinner.com's Women Who Rock Right Now. She began working on a small side project called Native Korean Rock and the Fishnets with fellow NYC musicians in 2008.
In 2009, she contributed backing vocals, screaming animal sounds, and noises to the songs "Gemini Syringes", "I Can Be A Frog", and "Watching the Planets" on The Flaming Lips album Embryonic. In 2011, she contributed vocals on the song "Pinky's Dream" on the David Lynch debut album Crazy Clown Time. In 2012, she collaborated with experimental rock group Swans on the song "Song for a Warrior" on their album The Seer. She also lends vocals to the song "GO!" on Santigold's 2012 album Master of My Make-Believe.
On the collaboration project with N.A.S.A. on The Spirit of Apollo, she appears on the track "Strange Enough", together with Ol' Dirty Bastard and Fatlip. O's vocal approach has been described as "ethereal" and has been described as "yelping" at times. She described her approach:
We still have to grab people by the collar ... We put out a record every three years now; we could easily be forgotten. If you look at a lot of our peers that we came up with, a lot of them have disappeared. — Karen O in the Los Angeles Times, 2009
O has also collaborated with James Iha on his second solo LP Look to the Sky in 2012. In 2014, Target used a version of the song "A Marshmallow World" sung by O. On May 5, 2015, a song by Karen O in tribute to Nellie Bly was used as part of a Google doodle commemorating Bly's 151st birthday. The same year, Microsoft approached O to pen a theme song for Square Enix's game Rise of the Tomb Raider, which resulted in the song "I Shall Rise".
In an interview with the New York Times, she discussed how being biracial led her eighth-grade peers to treat her as a "novelty," marking a "brutal but also pivotal" experience that "forced her to identify as a weirdo, which led her straight to rock ’n’ roll."
“My rebel streak comes from my mum. She’s Korean and when she started dating my dad, who’s Polish-American, it was highly controversial. He was teaching English in Seoul and was on a bus that she walked on to. It was love at first sight for him: he took the same bus every day, hoping she’d walk back on. She had all these wealthy young Korean suitors, but chose the broke Polish-American guy.” Despite being biracial, she continues to become proud of her cultural heritage in the midst of its controversy.