O'Mara died in a nursing home in England after fighting a short illness, her agent told the AP.
Born Frances M. Carroll in Leicester, O'Mara made her stage debut in a production of The Merchant of Venice in 1963. She went on to appear in numerous British television shows, including the BBC drama The Brothers from 1975-1976.
O'Mara became best known for playing the scheming sister of Joan Collins' character on the American soap Dynasty. She also starred on Doctor Who in 1985 and 1987, and reprised her role for the 30th anniversary in 1993. Just prior to the show's recent 50th anniversary, she told Digital Spy that she'd love to return. "I have white hair now, but a lot of it, and I'm still very glamorous and so I won't disappoint, I hope!" she said. "I'll still be wearing the tight leather trousers and high-heel boots, regardless of what age! That's what the fans are expecting, so that's what you've got to give them."
O'Mara also starred in the '90s comedy series Absolutely Fabulous and more recently appeared in the 2012 stage adaptation of Agatha Christie's Death on the Nile.
"Tragic news about Kate O Mara , we had great fun on Dynasty when she played my sister Caress who Alexis insisted on calling CASSIE ! RIP," Collins tweeted upon hearing of the news.
"She was extraordinary," O'Mara's agent Phil Belfield said. "She had so much energy and vitality with a love for theatre and acting."
O'Mara was married and divorced three times and had one son, Dickson, who died in January 2013.