The X Factor host's news was announced during a segment on the spinoff show tonight (November 19), in which O'Leary had to guess which famous movie scenes the X Factor contestants were re-enacting.
O'Leary refused to give his final answer in the quiz to prevent the presenting pair from having time to reveal the news. However, at the end of the segment, Flack shouted: "We want to congratulate you... on your engagement!", prompting a round of applause in the studio.
When asked by Murs why he had taken so long to propose to Koppang, who he has been dating for ten years, O'Leary quipped: "Well, you don't want to rush these things."
The couple reportedly bought a London home worth £3.5m together in April.
> Dermot O'Leary: 'X Factor Wembley Arena final will be incredible'