The two cable hosts have traded numerous barbs in the press in recent years, with Olbermann frequently naming O'Reilly on his nightly 'Worst Person in the World' list on his former MSNBC chatshow.
Olberman has now admitted that he has a genuine dislike for O'Reilly, charging that the Fox News anchor often refuses to correct factual errors on his O'Reilly Factor telecasts.
"I've seen [O'Reilly] at Yankee Stadium, and every time I looked in his direction, he was looking at me and immediately looked away," Olberman revealed to The New York Times. "What I can't stand about him is that he never, ever says he is wrong. He is slower in correcting his mistakes than the Catholic Church was in acknowledging they got it wrong about Galileo."
Olbermann has also been very critical of his former MSNBC bosses, with whom he frequently clashed during his final years hosting Countdown for the cable channel.
The journalist chided MSNBC bosses for their reported attempts to appeal to political moderates by hiring Republican strategists like former GOP chairman Michael Steele as on-air contributors.
"There is a lot of desperation over there now, and it's completely justified," he insisted.
Olbermann returned to television on Monday to host a revived Countdown, which now airs weeknights on Current TV.
> Keith Olbermann enlists Moore, Burns for series
> Bill O'Reilly to guest in Rizzoli & Isles
Watch Keith Olbermann and Michael Moore discuss the military conflict in Libya on Countdown below: