[1] O'Sullivan boxed as an amateur while working as an electrician's mate and later while serving in the Royal Air Force during World War II, winning the ABA bantamweight title in 1947 and fighting in the European championships in Dublin.
He beat Jean Louas and Georges Mousse in his next two fights, before suffering the first defeat of his career in May 1948 when he was beaten on points by Stan Rowan in a British title eliminator.
In January 1949 he lost to Mickey MacKay after being disqualified for a low blow, but bounced back a week later, forcing European champion Guido Ferracin to retire in the fifth round of their fight.
[5][6] Wins over Ronnie Draper, Fernando Gagnon, Bunty Doran, and Belgian champion Michel Verhamme followed before he got a second shot at the British title, Rowan having vacated.
O'Sullivan forced Teddy Gardner to retire at the end of the ninth round with a cut eye at the Royal Albert Hall in December 1949 to become British bantamweight champion.