Bill O'Reilly, David Letterman High Five Over Iraq War (Video)

John Paul Filo/CBS

Bill O'Reilly and David Letterman joked around on the Late Show Thursday night -- a far cry from their prior, tense appearances.

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(In 2009, Letterman called him a goon; in 2006, he told him, "About 60 percent of what you say is crap.")

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Thursday, Letterman and O'Reilly debated Occupy Wall Street and the Tea Party, with O'Reilly telling him he made "an excellent point" and Letterman politely thanking him.

After O'Reilly admitted the war in Iraq "should not have happened in hindsight" because the country did not have weapons of mass destruction, Letterman stood up for a high five.

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"Up high! Come on, up high....come on, Billy! Come on! Come on!" Letterman enthused as O'Reilly shook his head.

"I'm not high-fiving you on a war! I'm sorry…." O'Reilly replied. "We're having a good conversation, sit down."

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Finally, O'Reilly stood up and gave him a high five.

"Yeah!" said Letterman, before joking: "No wonder Cheney won't come on your show. Mr. Grouch!"