The X Factor host swapped his sharp suits for floral prints to promote the National Television Awards in this week's Heat.
"I'm a huge fan of Mary's and I feel good about her floral bomber jacket!" he told the magazine.
Asked who he wants to see win an NTA this year, O'Leary added: "Claire Balding. I think she's just so effortless. She and Gabby Logan did a great job on the Olympics."
See an exclusive photo shoot of Dermot dressed as Mary Berry, Ian Beale and Miranda in this week's Heat, out now.
Watch Dermot O'Leary host the National Television Awards live on ITV1 on January 23, from 7.30pm.