Between these appearances in "Doctor Who", she auditioned for the role of Sable Colby on the American primetime soap "The Colbys", which was a spin-off of hit soap opera "Dynasty". Eventually, O'Mara had declined the role since she was under contract with London stage play "Light Up the Sky" at the Old Vic Theater, and the role went to Stephanie Beacham. Shortly after, she was offered to play Caress Morell on "Dynasty". As the sister of Alexis Colby (Joan Collins), O'Mara appeared in 17 episodes of the sixth season and 4 episodes of the seventh during 1986. "We had a tremendous bitchy tension between us", the actress recalled about performing opposite Collins. “My character Caress was like an annoying little mosquito who just kept coming back and biting her.” O'Mara disliked living in California, preferring the change of seasons in Britain, and to her relief was released from her five-year contract after Collins told the producers that having two brunettes in the series was a bad idea. After returning to the UK, she was cast as another scheming villain, Laura Wilde, in the BBC soap "Howards' Way" (1989–1990).