They will replace Michael Buble as hosts of next month's show after the Canadian singer pulled out to care for his ill three-year-old son.
The Brit Awards take place at the O2 in London on 22 February.
"The Brits is an iconic show that I've watched and attended many times," said X Factor presenter O'Leary.
Emma, who is currently presenting The Voice UK, said: "To present the Brits is an honour but to be doing it alongside one of my absolute favourites, Mr Dermot O'Leary is just bonkers. Bring on the Brits 2017."
Robbie Williams, Little Mix, The 1975 and Emeli SandÃ© have already been confirmed to perform at the event.

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Hosszu's time of four minutes 26.36 seconds took more than two seconds off the previous best set by China's Shiwen Ye at London 2012.
Scotland's Hannah Miley, 26, finished fourth after being pipped to bronze by Spain's Mireia Belmonte Garcia.
Fellow Briton Aimee Willmott, 23, finished seventh.
Miley, ranked second in the world behind 27-year-old Hosszu, said she had "nothing left" to stop Belmonte overtaking her on the final leg.
"I could see her. I knew she was coming back at me. I gave everything I had," Miley, who won gold at the 2014 Commonwealth Games, told BBC Sport.
"It is such a mixture of emotions. I was so close to getting it. It is happy and disappointment all coming together."
Rebecca Adlington, double Olympic gold-medal winner
Find out how to get into swimming with our special guide.
"I am so angry. I was like, 'Hannah what were you doing?' In the last 50 metres, she did not breathe and look at Belmonte once.
"If you know someone is coming up, you can give that extra tiny bit if you breathe and think 'she is right there'.
"I am devastated for her because that was her opportunity. All she had to do was breathe once."
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Dermot O'Leary and Emma Willis are to present this year's Brit Awards, organisers have announced.
[NEXT_CONCEPT]
Hungarian swimmer Katinka Hosszu set a remarkable new world record as Britain missed out on a medal in the women's 400m individual medley at Rio 2016.