Her earliest television appearances, in the 1960s, included guest roles in "Danger Man", "Adam Adamant 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.