KEITH Olbermann is re turning to his sports casting roots – at least on Sundays.

The outspoken MSNBC commentator has been added to NBC’s Sunday-night NFL show, “Football Night in America,” as a co-host with Bob Costas and Cris Collinsworth.

“It just flowed together naturally,” Olberman said yesterday during a conference call with reporters – adding that he’ll likely keep his left-leaning political opinions mostly confined to his MSNBC show, “Countdown.”

“It’s a football show,” Olbermann says of his new NBC Sports gig. “I’m not going in there with any agenda. I’ve been a sportscaster longer than I’ve been a commentator and I think I know the difference when to do stuff like that and when not to.”

Olbermann had been in negotations with NBC for a new contract that would focus primarily on news, he said.

A few weeks ago, NBC boss Jeff Zucker approached NBC Sports chairman Dick Ebersol with the idea of bringing Olbermann back to sports – while keeping him in news at the same time.

“They both thought that football sounded like an ideal fit,” said Olbermann. “There’d be time to do it, there wouldn’t be travel involved.”

Jerome Bettis and Tiki Barber are the show’s analysts.