Her earliest television appearances, in the 1960s, included guest roles in Danger Man, Adam Adam Ant Lives!, The Saint, Z-Cars and The Avengers. In 1970, she appeared in two Hammer Studio horror films: The Vampire Lovers and The Horror of Frankenstein. In the former, she had an erotically charged scene with Ingrid Pitt, in which O'Mara was meant to be seduced; the two women were left laughing on set, however, as Pitt's fangs kept falling into O'Mara's cleavage. O'Mara's work in The Vampire Lovers impressed Hammer enough for them to offer her a contract, which she turned down, fearful of being typecast.