Dermot O'Leary is a British television and radio presenter. He established himself as a fixture as presenter of Big Brother's Little Brother on Channel 4 and has since moved on to The X Factor over on ITV. He has also presented on the BBC and has his own radio show on BBC Radio 2.

O'Leary was born and grew up in Colchester to Irish parents. He attended primary school in Marks Tey and later St. Benedict's College (Roman Catholic) in Colchester, graduating with two General Certificates of Secondary Education, and Colchester Sixth Form College. O'Leary studied at Middlesex University, achieving a degree in Media and Television with Politics. 

O'Leary started as a disc jockey at Radio Essex. After leaving due to unexplained circumstances he became a runner on the TV show Light Lunch with Mel Giedroyc and Sue Perkins and then presenter at Channel 4, being part of the original presenting line-up of the channel's T4 strand, before moving on to present Big Brother's Little Brother (the companion show to Big Brother) on E4 from 2001 onwards.

As of 2006, the show became less popular due to the huge success of fellow Big Brother sister show Big Brother's Big Mouth. O'Leary shares the same talent agency as Russell Brand and Davina McCall - John Noel Management. On 27 November 2007, it was announced that Big Brother: Celebrity Hijack was to be O'Leary's last series of Big Brother, and that he would not return in summer 2008. After seven years he made his final Big Brother appearance on 28 January 2008. A special 'Best Bits' trail was played.

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. Russell Brand fever was blamed for that move. The show is now hosted by Ben Shephard.

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. Speaking after the announcement he said: "I was very flattered to be offered the job to host such a hugely successful show. I'm really looking forward to working with Simon Cowell and the team on the kind of Saturday night entertainment show I grew up watching." While presenting the show he reportedly upset Celine Dion among other things, later saying he "literally had to shut her up".

In March 2009, O'Leary introduced Michael Jackson to a large audience of fans in London in what turned out to be Jackson's final public appearance before his death, with the two sharing an affectionate hug.

In May 2009, O'Leary became the new host of Matt Lucas and David Walliams' Rock Profile.

In March 2010, O'Leary hosted a spinoff edition of the BBC One show Question Time, which aired on BBC Three. The show was aimed at first time voters, and in June 2010, O'Leary hosted Soccer Aid on ITV 1.