in The Stranger's Hand with Alida Valli and Trevor Howard.

One of his earliest television roles came on an episode of Sherlock Holmes titled  The Unlucky Gambler,  which aired when he was just 11.

He played a small role in Carry On Teacher (1959) and starred as the younger brother of Audrey Hepburn's Gabrielle in The Nun's Story that same year. In the early '60s, he appeared in two Cliff Richard films, The Young Ones (1961) and Wonderful Life (1964).

For most of the 1960s, O'Sullivan focused on television with credits including Emergency Ward 10, Redcap, Danger Man and No Hiding Place. Among some of his best-known appearances aside from Man About the House and Robin's Nest are playing the titular character in 1979's Dick Turpin series as well as 1971's Doctor at Large and 1972's Doctor in Charge.

In the '80s, he starred in Me and My Girl and in 1991 led ITV's Trouble in Mind.

O'Sullivan also had a relationship with his Man About the House co-star Thomsett and later, with Wyatt, with whom he had his son.