The 'Troublemaker' star - who, of course, launched his own career on X Factor - had been widely rumoured as Gary's special guest so today's confirmation wasn't a surprise.
Olly tweeted: "Hey all, so there have been loads of rumours about it & I can confirm that YES I'll be @GaryBarlow 's Guest Judge on this year's @TheXFactor."
And, joking we could be seeing a very different side to him, added: "And be prepared.... #TheNewMrNasty lol ;)"
As you'd expect, Olly's fans were rather thrilled by the news, with responses including: "Gary Barlow is one very lucky person having your help!"
For Olly, this will be his third change of role on X Factor.
He started as a contestant on the sixth series in 2009, eventually finishing runner-up to Joe McElderry. He then returned as a co-presenter on Xtra Factor alongside Caroline Flack, but left earlier this year to focus on his music. Now, he's returning as a guest judge.
What's next? A role at the judging table?! We wouldn't say no to that!
X Factor will begin its boot camp sessions this weekend, followed by judges' houses when each judge will fly their category to an exotic location where they'll decide who to bring through to live shows.
Nicole Scherzinger took her category to Antigua, but it hasn't been confirmed yet where Gary, Louis Walsh and Sharon Osbourne have jetted with their acts. It's been rumoured, however, that Gary chose New York, Louis the south of France and Sharon her adopted hometown of LA.
Watch Olly's music video for 'Right Place Right Time' below: