O'Leary joined BBC Radio 2 in 2004 and currently presents the Saturday morning breakfast show[1] and since April 2022 Alternative Sounds of the 90s.

[5] He attended primary school in nearby Marks Tey and later joined St Benedict's Catholic College in Colchester.

[8] When he was in his late teens, he played American football for the Colchester Gladiators and the Ipswich Cardinals, where he wore the number 32.

On 29 March 2007, it was announced that O'Leary would replace Kate Thornton as the new host of ITV's The X Factor for at least two series.

I'd like to wish the team all the best for the future, especially whoever takes over from me.O'Leary was replaced by Olly Murs and Caroline Flack, who previously co-hosted The Xtra Factor, the behind-the-scenes companion programme, together in 2011 and 2012.

On 29 March 2016, O'Leary returned to The X Factor as a presenter, replacing Murs and Flack, for the show's 13th series.

In September 2013, O'Leary hosted BRITs Icon: Elton John, a one-off music celebration for ITV.

In January 2014, O'Leary filled in for Phillip Schofield for a week when he guest presented This Morning opposite Holly Willoughby.

In November 2020, it was announced that Eamonn Holmes and Ruth Langsford had been dropped from their Friday slot on This Morning after 14 years.

In March 2014, O'Leary made a return to Channel 4, where he presented the Live From Space season for three consecutive nights.

In August 2006, O'Leary signed a deal to present the game show 1 vs. 100, which aired on Saturday nights on BBC One in combination with The National Lottery Draws.

In May 2009, O'Leary became the presenter of Matt Lucas and David Walliams's show Rock Profile on BBC.

[17] He also presented Dermot Meets..., a series which saw him interview the likes of David Cameron, Gordon Brown, and Nick Clegg.

O'Leary joined London's indie rock station XFM in 2001, firstly presenting weekday mid-mornings from 10 am to 1 pm, before moving to a Saturday evening show in mid-2002, where he remained until late 2003.

He is one of nine presidents of Better Planet Education[27] and is a patron of the international children's film festival CineMagic, a registered charity for young people based in Belfast.

[28] He has campaigned on behalf of Make Poverty History, and visited Sierra Leone with CAFOD accompanied by his father.

In 2003, he played in a charity match for the Colchester Gladiators as a punt returner, helping to raise £2,500 for the Barnardo's children's fund.

In March 2014, along with Jeremy Kyle, Bill Bailey, John Prescott, Richard Osman, Rizzle Kicks, Louis Smith, Levi Roots, and Ricky Wilson, O'Leary went commando for charity to raise awareness of testicular cancer.

[31] For Red Nose Day 2015, O'Leary participated in a 24-hour dance to raise money for Comic Relief.