The star was a household name in the 1980s, with roles in Dynasty, Howards' Way, Triangle, and as a scheming Time Lady in Doctor Who.
But she was best known as Alexis Colby's sister Cassandra "Caress" Morrell in the US soap. She died in a Sussex nursing home after a short illness yesterday.
Her agent Phil Belfield described her as an 'extraordinary' woman. He added: "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."
She played the villainous Rani in Doctor Who and had talked about reprising her role. The show's former co-star Bonnie Langford said: "So sad to hear that the wonderful Kate O'Mara has gone. Treasured memories."
O'Mara's first TV roles were in the 1960s. But it was in the 1980s that she landed her best-known roles.
She also played a ruthless businesswoman in BBC drama Howards' Way.