He was known as the "voice of racing". In a television interview before his fiftieth and last Grand National in 1997 he revealed that his commentary binoculars came from a German submarine. He was knighted the same year - the only sports broadcaster at that time to have been bestowed that honour by the Queen.
O'Sullevan is a successful racehorse owner, including Be Friendly who won the King's Stand Stakes at Ascot, and Prix de l'Abbaye de Longchamp. He was twice successful in the Haydock Sprint Cup in 1966 and 1967 (and would probably have won the 1968 renewal had the meeting not been abandoned due to waterlogging).
Another popular horse he owned was Attivo, whose victory in the 1974 Triumph Hurdle at the Cheltenham Festival was described by O'Sullevan as the most difficult race to call. After passing the line, O'Sullevan uttered “And it’s first Attivo ... trained by Cyril Mitchell ... ridden by Robert Hughes ... owned by Peter O'Sullevan”. Attivo also won the Chester Cup and the Northumberland Plate during the 1970s. O'Sullevan's final race commentary came at Newbury Racecourse for the 1997 Hennessy Cognac Gold Cup, and he visited the winners' enclosure as a winning-owner in the race which followed courtesy of Sounds Fyne's victory in the Fulke Walwyn Chase. He was succeeded by Jim McGrath.
Since his retirement, O'Sullevan has been actively involved in charity work fund-raising for causes which revolve around the protection of horses, including the ILPH, International League for the Protection of Horses.
The National Hunt Challenge Chase Cup (ran at the Cheltenham Festival) was named for him in 2008 to celebrate his 90th birthday.