O'Sullivan was born in Lombardstown, County Cork and came from a family that had a long association with national hunt racing.
His father, William, won the Foxhunters' Chase in 1991, on Lovely Citizen, a horse owned and bred by his grandfather Owen and trained by his uncle Eugene.
[6][7] O'Sullivan ended his breakthrough campaign by being named Irish Champion conditional jockey with 32 winners.
[9] On 6 February 2025, O'Sullivan was on board Wee Charlie in a handicap chase at Thurles when he was severely injured after being one of three fallers at the final fence.
[10] He received treatment at the course and was later transferred via air ambulance to Cork University Hospital where he was put into an induced coma.