The duo told how the seemingly timid former Asda worker used to terrorise the flamboyant and outspoken Rylan Clark before his exit.
Speaking to Digital Spy and other journalists, O'Leary commented: "One of my favourite things is watching how [contestants] grow, and Jahmene especially is a cheeky little bugger now isn't he?"
"A nice cheeky little bugger," Xtra Factor host Flack clarified. "But I really like the fact that his confidence has grown, because he was one of those people that I really liked from the start and... that hasn't changed at all."
Jahmene Douglas and Rylan Clark with their mentor Nicole Scherzinger:
O'Leary continued: "I love him more [now]. I was really worried about him to start with because I just thought, 'How's this guy gonna cope?' He's really funny.
"He used to do prank calls on Rylan. He'd ring Rylan up at four in the morning and just leave a message going, 'You have no new messages', and just put the phone down."
Both O'Leary and Flack named Ella Henderson as the act they would have wanted to see make the final, with the former adding: "I think we all were quite excited about what she'd do when she got here."
The pair also told Digital Spy that they are "thrilled" to see Christopher Maloney in the final.