[1] He rode his first winner on Johann Zoffany, trained by his father, at Leopardstown Racecourse on 28 May 2009 and gained his first classic success when Roderic O'Connor won the 2011 Irish 2,000 Guineas.

[10] In March 2016 O'Brien announced that he would be stepping down from race-riding to concentrate on his new career as a trainer.

[11] O'Brien began his training career at Owning Hill, County Kilkenny.

He made an immediate impact when Intricately, ridden by his younger brother Donnacha, won the Moyglare Stud Stakes in 2016.

He went on to win the Melbourne Cup with Rekindling in 2017 and with Twilight Payment in 2020; the Irish Derby with Latrobe in 2018; and the Pretty Polly Stakes with Iridessa in 2019.