Possibly his earliest television work was the part he played in the Sherlock Holmes episode, "The Unlucky Gambler", broadcast on 18 July 1955.

Around the same time, he was cast in the role of Pierre van der Mal in an early scene of The Nun's Story (also 1959), playing the younger brother of Gabrielle (Audrey Hepburn).

Also around that time, he had a leading role in an episode of the Sapphire/ITC series The Four Just Men ("The Man with the Golden Touch", 1959), as Neapolitan street urchin Pietro, who foils a robbery.

[3] In the 1963 blockbuster Cleopatra, he appeared as Pharaoh Ptolemy XIII, the younger brother of the title character played by Elizabeth Taylor.

[1] For the remainder of the 1960s, O'Sullivan was a jobbing actor appearing in such TV series as Dr Syn: the Scarecrow, Emergency Ward 10, Redcap, Danger Man, No Hiding Place, Dixon of Dock Green and Strange Report among others, until he was offered the role of Lawrence Bingham in the LWT sitcom Doctor at Large (1971), a role which continued in the later Doctor in Charge (1972–73).

To tie in with the series, O'Sullivan wrote a recipe book called Man About the Kitchen, and a sequel Roastin’ with Richard which were published in 1980.

He then played the widower Simon Harrup in the sitcom Me and My Girl, broadcast from 1984 to 1988, co-starring Tim Brooke-Taylor and Joan Sanderson, also produced by LWT.

He also appeared in a one-off comedy-drama The Giftie, shown on Channel Four in 1988, in which he and a friend discovered a photocopier at work that could duplicate living copies of themselves, unwisely doing so, and predictably leading to mistaken identities and chaos.