She was best known for her role in Dynasty as the scheming Cassandra Morrell. She made her first appearance on Television in 1964 in the TV show No Hiding Place.
Her agent Phil Belfield said: "She was extraordinary, she had so much energy and vitality with a love for theatre and acting.
"A shining star has gone out and Kate will be dearly missed by all who knew and have worked with her."
Colin Baker, who she starred against in The Mark of the Rani commented on her passing on Twitter:
Oh my goodness. Kate O'Mara is no longer with us. Sad sad news. A delightful, committed and talented lady and actress. We are the poorer
— Colin Baker (@SawbonesHex) March 30, 2014
Sources: BBC News, ITV, Express.