Aidan O'Brien

Irish Thoroughbred trainer

Aidan Patrick O'Brien (born 16 October 1969 in County Wexford, Ireland) is an Irish horse racing trainer. Since 1996, he has been the private trainer at Ballydoyle Stables near Rosegreen in County Tipperary for John Magnier and his Coolmore Stud associates. He is widely acknowledged as one of the greatest horse racing trainers of all time.

Aidan O'Brien was one of six children of Denis O'Brien (died 1 December 2008) and his wife Stella (née Doyle). Denis was a farmer and small scale horse trainer in the townland of Killegney, near Poulpeasty, in County Wexford, where Aidan grew up. Aidan O'Brien attended Donard National School, located less than a mile from his parents' home. He subsequently attended secondary school at Good Counsel College, New Ross, also located in County Wexford. O'Brien is a member of the Pioneer Total Abstinence Association, meaning that he does not drink alcohol. O'Brien first started working professionally with horses at P. J. Finn's racing stables at the Curragh, County Kildare, and then with Jim Bolger at Coolcullen, County Carlow. O'Brien and Anne Marie have four children, with Joseph, Sarah, Anastasia and Donnacha all jockeys. Joseph became apprenticed to his father and rode his first winner shortly after his sixteenth birthday, on Johann Zoffany at Leopardstown on 28 May 2009. In 2012 O'Brien and Joseph, 19, became the first father son/trainer jockey combination to win The Derby, with Camelot. As of 2023, Aidan O'Brien is the most successful Epsom Derby trainer of all time with 9 wins.

O'Brien is backed by the Coolmore operation. The Coolmore operation is renowned for high priced yearling purchases at the Keeneland September Yearling Sales and elsewhere.