The talent show host said that the American singer "gets" the British style of humour and has fitted in well with her fellow judges Gary Barlow, Tulisa Contostavlos and Louis Walsh.
"She's only done one city so far, she gets our sense of humour," he told Daybreak. "She spends so much time with Lewis [Hamilton] I suppose.
"I miss Kelly [Rowland] obviously, but Nicole's brilliant. She's serene. The camera loves her."
The X Factor auditions are currently in Cardiff, which will be the last stop before the bootcamp stage begins.
O'Leary also revealed that the talent has been high so far in this year's ninth series of the ITV1 singing competition.
"The talent has been off the scale. There's been really good stuff this year."
Digital Spy readers recently gave a mixed response to this year's judging panel.
The ninth series of The X Factor will begin in the autumn on ITV1.