E! News said O'Brien wore the trendy jeggings on his TBS show after promising fashion guru Tim Gunn Wednesday he would do so.
"Men don't wear jeggings," O'Brien argued with Gunn after he was informed some males are, indeed, walking around in them. "What if I wore, for a whole show, what if I wore jeggings one night? Seriously?"
Shortly after beginning his monologue for Thursday night's show, O'Brien replayed the clip from his Gunn interview and declared: "I have a motto on this show. If I say I'm gonna do something, I do it!"
He then doffed his suit pants and showed off his form-fitting jeggings.
As he strutted around the stage and tried to continue his monologue, the crowd laughed heartily.
"I fought my way back onto television for this," O'Brien joked. "This is not why hi-def was invented."
Later in the program, he added: "I will say this, they are incredibly comfortable ... They're horrifying to look at, but they feel … I feel like I'm being molested."