Sean Patrick Paul O'Sullivan, CM (January 1, 1952 – March 9, 1989) was a Canadian politician who left politics and became a Roman Catholic priest.
Born in 1952 in Hamilton, Ontario, O'Sullivan showed an interest in politics at a young age: when he was 11 years old he worked for Ellen Fairclough in her election campaign and met John Diefenbaker for the first time. He was educated at Bishop Ryan Catholic Secondary School and Brock University in St. Catharines. In 1970, at age 18, he was chosen as president of the Young Progressive Conservatives of Canada.
In 1971, Diefenbaker appointed O'Sullivan as his executive assistant, and in the 1972 election, the 20-year-old O'Sullivan was elected the Progressive Conservative MP for the riding of Hamilton—Wentworth, becoming the youngest member of the Canadian House of Commons ever at that point (in 1974, Liberal Claude-André Lachance became the youngest-ever MP, and New Democrat Pierre-Luc Dusseault beat that record in 2011). He rose quickly through the ranks of the party and was seen as a future cabinet minister or even leader. He was re-elected in 1974.
However, in 1977, O'Sullivan surprised many political observers when he announced that he was leaving politics to enter the Roman Catholic priesthood, eventually becoming Director of Vocations for the Archdiocese of Toronto, as well as publisher of The Catholic Register newspaper.
O'Sullivan learned he was suffering from leukemia in 1983. After treatment, the disease was in remission and he resumed his busy career. In 1986, he was made a Member of the Order of Canada. With the return of his illness in 1989, O'Sullivan entered Princess Margaret Hospital in Toronto for a bone marrow transplant. He died there on March 9, 1989, aged 37.