O'Leary promised that Osbourne, who is replacing singer Tulisa, would bring a "kind of anarchy" to the show.
The judges - who also include Gary Barlow, Nicole Scherzinger and Louis Walsh - described what they were looking for in this year's contest.
The X Factor judges – including comeback judge Sharon Osbourne – are back together ahead of the first filmed auditions of the new series.
Osbourne, who was on the show from 2004 to 2007 and replaces Tulisa Contostavlos, will feature alongside Louis Walsh, Nicole Scherzinger and Gary Barlow.
Although preliminary auditions have already taken place, the judges start their first day on the programme today.
Original X Factor judge Sharon Osborne is to return to the show for its 10th anniversary.Read the full story ›
The Black Sabbath front man Ozzy Osbourne has confirmed that he is not divorcing his wife Sharon following reports that the couple were living apart.
The British singer confirmed on his Facebook page that over the past year he has been "drinking and taking drugs". He apologised to his family and said that he was pleased to say he "has been sober for the past 44 days".
His son Jack Osbourne also rubbished reports that his parents were separating. Writing on his Twitter account he said "Last time I check (sic) a lot of British news papers weren't amazing sources of accurate information. Moving on..."
Sharon Osbourne has addressed her daughter's health on American TV show 'The Talk'.
She said they were lucky it happened on camera, as it meant the doctor could physically see what happened to Kelly.
She said: "She's on medication, she's fine."
Sharon added that doctors still do not know why Kelly had the seizure.
Chrystalla Spire told ITV News she, "couldn't be more pleased, " that Sharon Osbourne had spoken about her double mastectomy.
She said; "it's a tremendous fillip to this area which is still overcome by taboo."
Ms Spire founded and runs a group to support women who have been through similar experiences.
Speaking about her own decision to have a double mastectomy she said, "it was a very easy decision to make, I discussed it with my daughters. And we really didn't take very long to make the decision and it was one that I was always very pleased I had made.
Prior to the surgery for her double mastectomy Sharon Osbourne also learned her breast implants had leaked into her stomach wall, which has now put her off cosmetic surgery.
"Sometimes I'll see a photo and I'll think, 'My face looks plastic'; it can look so unnatural from certain angles. Now I am definitely, definitely done," Osbourne told Hello! Magazine.
"You can't buy your youth back, no matter how much money you've got. I won't be going under the knife again."
Daybreak's Health Editor Dr Hilary Jones has told Daybreak that Sharon Osbourne "had one of the faulty genes" which would have made her much more susceptible to cancer.
Sharon Osbourne has revealed that the decision to have a double mastectomy was a 'no brainer' after taking the agonising decision to opt for surgery. Daybreak's Ross King reports: