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The X Factor presenter has proposed to Dee Koppang
O'Leary and his girlfriend of 9 years became engaged at the beginning of November during a trip to New York but kept the news secret.
Dermot's X Factor colleague, Caroline Flack, let the cat out of the bag during the talent show, when she congratulated O'Leary and Koppang live on TV.
"We want to congratulate you... on your engagement!" said Flack.
When asked why it took him so long to propose to his girlfriend, O'Leary said, "Well, you don't want to rush these things."
The TV presenter later took to his Twitter to say thank you to his well-wishers, saying he was "A very lucky boy".