Aidan O'Brien was one of six children of Denis O'Brien and his wife Stella. 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. Aidan O'Brien is married to Anne-Marie. Anne-Marie's father, Joe Crowley trained horses at Piltown, County Kilkenny, where his tenure was interrupted in quick succession by his daughter Anne-Marie, his son-in-law Aidan O'Brien and then another daughter, Frances Crowley. Joe then renewed his own training licence for some years before retiring. Aidan O’Brien was champion Irish National Hunt trainer in the 1993/4 season and went on to lift the title for the next 5 consecutive seasons. His most successful horse during this time was the famed Istabraq. In 1996 he was approached by John Magnier to train at Ballydoyle. For a number of years he retained his Piltown yard. 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 2020, he is the most successful Epsom Derby trainer of all time with 8 wins.
Desert King won the Irish 2,000 Guineas to give Aidan O'Brien his first classic success.
2001 Season
Aidan O'Brien was crowned England's champion trainer, the first Irishmen to achieve the feat since Vincent O'Brien in 1971.
2004 Season
2004 Proved to be a disappointing year for O'Brien. It started off with the removal of Michael Kinane as stable jockey. Kinane was replaced by the young Jamie Spencer who had developed a good reputation riding in the UK. However the year ultimately proved disappointing on the track. Spencer was replaced after a string of self-confessed mistakes on Powerscourt in the Arlington Million and on the same horse again in the Breeders' Cup Turf.
The start of the 2006 flat season saw George Washington confirm his juvenile promise by easily winning the 2,000 Guineas on Newmarket's Rowley Mile to give O'Brien his fourth win in the race. While George Washington disappointed in the Irish equivalent, he redeemed himself in the Queen Elizabeth II Stakes at Ascot. Dylan Thomas also made his mark in the top middle distance races in Europe. After going close in the Epsom Derby, he was found to be more effective under a patient Kieren Fallon ride in the Irish Derby. Dylan Thomas also showed his battling qualities in the Irish Champion Stakes, where he beat the Champion mare Ouija Board by a neck. Other 2006 highlights came through Alexandrova who dominated the middle distance fillies classics taking in The Oaks, Irish Oaks and Yorkshire Oaks. The end of season saw the juvenile Holy Roman Emperor take the Prix Jean-Luc Lagardère on Arc day at Longchamp. In one of the races of the season he was beaten by a head in the Dewhurst Stakes by European Champion 2-year-old Teofilo. Much anticipation centred on the expected rematch of these two colts in the 2,000 Guineas at Newmarket the following spring. Aidan finished the season with prizemoney of over €6.5m in Ireland and Great Britain. The season was marked however by the emerging race fixing allegations involving stable jockey Kieren Fallon. On 3 July 2006, Fallon was charged along with 7 other people for conspiring to defraud clients of the internet betting exchange Betfair. Fallon was one of 28 people answering bail at Bishopsgate police station in the City of London in connection with the inquiry. In a released police statement, he was described as being charged with "offences relating to allegations of fixing the outcome of horse races between 1 December 2002 and 2 September 2004, and money laundering". He was amongst three jockeys charged, along with Darren Williams and Fergal Lynch, all of whom had their licences to ride in Britain suspended, effectively under the principle that they would be guilty until found innocent. Fallon's problems were further compounded when on 29 November 2006 the French racing authority France Galop suspended Fallon for six months after he tested positive for a metabolite of a banned substance.
2007 season
The end of the 2006 season had apparently left Ballydoyle with multiple options for the following year's classics. However the opening part of the season was left in turmoil after the premature retirement of Holy Roman Emperor. The previous year's champion miler George Washington proved almost infertile, ultimately siring only one foal. As a late replacement, Holy Roman Emperor was sent to Coolmore Stud. Plans had to be drastically changed with O'Brien saying "all our eggs were in one basket" in relation to the opening classic of the season the 2,000 Guineas. As a result, the season began relatively quietly as far as O'Brien's classic crop was concerned. However signs of changing fortunes would emerge on the day of the Poule d'Essai des Poulains when Astronomer Royal came with a late run to give Ballydoyle a 1–3–4 finish. Group 1 success was seen in the older horses through the previous year's Irish Derby winner Dylan Thomas when he won the Prix Ganay. The mid summer was highlighted by Scorpion winning the Coronation Cup in an O'Brien 1–2 with stable mate Septimus. The stable also dominated the St. James's Palace Stakes and the Irish Derby with Excellent Art and Soldier of Fortune respectively leading home 1–2–3 for O'Brien horses. O'Brien finished Royal Ascot as top trainer with four winners including Yeats who retained his Ascot Gold Cup and Henrythenavigator who set the juvenile standard for the early part of the season by winning the Coventry Stakes. Kieren Fallon returned to retainer duties in both Ireland and France, although his ban in Britain was still being enforced pending the result of his trial in relation to fraud in racing. Peeping Fawn managed to learn from her second place in The Oaks by taking the Audi Pretty Polly Stakes at the Curragh and eventually overturning form, albeit on softer ground, with Light Shift in the Irish Oaks. The season of 2007 also saw Aidan land the one big European race that had so far eluded him throughout his illustrious career when Dylan Thomas won the Prix de l'Arc de Triomphe under Kieren Fallon. The season however ended on a very sad note after George Washington was put down after breaking down in the Breeders Cup Classic.
2008 Season
During the off season Johnny Murtagh was appointed stable jockey after Kieren Fallon's acquittal on race-fixing charges was quickly followed by a second drugs suspension. However the introduction of Johnny Murtagh to Ballydoyle was to prove a baptism of fire and assisted Aidan in what was to be a vintage Ballydoyle year, equalling his previous best Group 1 tally of 23. As a team Aidan and Johnny were to dominate the Irish classic scene taking all five Irish Classic Races thus being the first trainer to do so since 1935. He again was named Champion trainer in Britain taking his second title in a row. He equalled former Ballydoyle resident Vincent O'Brien's record of 6 winners at Royal Ascot, four of these at the highest level including Yeats' record-equalling third consecutive Ascot Gold Cup. Henrythenavigator won both the Irish and English 2000 Guineas, and followed up with the St James's Palace Stakes. Duke of Marmalade won 5 times at group 1 level and was considered by many to be the Champion older male horse in Europe at middle distances. Other highlights through the year were provided by Halfway to Heaven, Frozen Fire, Mastercraftsman, Mount Nelson, Fame and Glory, Septimus and Moonstone.
2016 Season
Aidan O’Brien won a record 7 races at Royal Ascot in 2016.
2017 season
O'Brien set a new world record by training 26 Group or Grade 1 winners in one calendar year when Saxon Warrior won the Racing Post Trophy at Doncaster, which beat the US-based Bobby Frankel's 2003 mark of 25. By the end of the year O'Brien extended the record to 28.
Coolmore Stud
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.
National Stakes – – Desert King, King of Kings, Beckett, Hawk Wing, One Cool Cat, George Washington, Mastercraftsman, Power, Gleneagles, Air Force Blue, Churchill
1000 Guineas – – Virginia Waters , Homecoming Queen , Minding, Winter, Hermosa , Love
2000 Guineas – – King of Kings, Rock of Gibraltar, Footstepsinthesand, George Washington, Henrythenavigator, Camelot, Gleneagles, Churchill, Saxon Warrior, Magna Grecia
Queen Elizabeth II Stakes – – George Washington, Rip Van Winkle, Excelebration, Minding
St. James's Palace Stakes – – Giant's Causeway, Black Minnaloushe, Rock of Gibraltar, Excellent Art, Henrythenavigator, Mastercraftsman, Gleneagles, Circus Maximus
Vertem Futurity Trophy – – Saratoga Springs, Aristotle, High Chaparral, Brian Boru, St Nicholas Abbey, Camelot, Kingsbarns, Saxon Warrior, Magna Grecia
