The X Factor host will marry long-time girlfriend Dee Koppang next week.
He has invited the boyband to the ceremony, along with several other friends connected to The X Factor, reports The Mirror.
Koppang has stated that the ceremony will be a large old-fashioned affair.
"It's going to be the full works, in a church because Dermot's Irish Catholic," she said.
However, the couple will be postponing their honeymoon, as it clashes with The X Factor. They will head off for a mini-break after the wedding, followed by a longer holiday next year.
O'Leary confirmed his engagement live on The Xtra Factor last year.
The TV presenter was crowned 'Most Stylish Man' at the GQ Awards earlier this week.
Watch Digital Spy talking to Dermot O'Leary about The X Factor below: