Aidan Patrick O'Brien (born 16 October 1969 in County Wexford, Ireland)[1] 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.

Denis was a farmer and small-scale horse trainer in the townland of Killegney, near Poulpeasty, in County Wexford, where Aidan grew up.

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 (Champion National Hunt Trainer during her brief time at the helm), his son-in-law Aidan O'Brien (who took over from his wife in 1993 but moved on to Ballydoyle in 1996) and then another daughter, Frances Crowley (who moved on to train on the Curragh for some years).

The end of season saw the juvenile Holy Roman Emperor take the Prix Jean-Luc Lagardère on Arc day at Longchamp.

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 among 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.

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 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.

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.

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.

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.

[10] 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.