O'Mara passed away in a Sussex nursing home, her agent announced.
O'Mara's many fans will remember her as Alexis Colby's cousin, the scheming Caress Morell, in 'Dynasty', and as The Rani in 'Doctor Who'.
She also starred in 'Howards Way' and played Patsy's sister Jackie in 'Absolutely Fabulous'.
Her agent Phil Belfield said, "She was extraordinary, she had so much energy and vitality with a love for theatre and acting."
Throughout her long career, she always stood out with her distinctive, sculpted cheekbones and intense gaze, a look that made her often cast as the villainess.
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O'Mara's first television roles were in the 1960s. But it was in the 1980s that she landed her biggest and longest-running roles. As well as Dynasty she starred as a ruthless businesswoman in BBC drama Howards' Way, which was dubbed the "Dallas of the seas".
In the 1990s, O'Mara starred in the comedy show Absolutely Fabulous as Jackie, the sister of Joanna Lumley's character Patsy. More recently she appeared in ITV soap Benidorm and a 2012 stage adaptation of Agatha Christie's Death On The Nile.