While most news channels focused on Sen. John McCain’s big night on Thursday, it was a battle of the B.O.s on Fox News.

Sen. Barack Obama, the Democratic presidential nominee, made a much-hyped appearance on “The O’Reilly Factor” to tangle with Bill O’Reilly. The talk-show host had been pushing so hard for an Obama visit, of course, his unrequited desire was a big topic at the secret meeting between Obama, NewsCorp head Rupert Murdoch and Fox News chief Roger Ailes.

Ever the showman, O’Reilly turned his new tape into a mini-series — only “Part 1″ aired Thursday (below) — and turned us commentators into previews for the remaining segments. But it was still good television.

Part 1 is about national security, with O’Reilly gingerly mugging as a salt-of-the-earth armchair general. He may pull down $9 million a year, but O’Reilly talks with the cadence and umbrage of any regular guy holding court at the bar — scrunching his face, interrupting, waving his hands and wondering whether Obama is tough enough to take on Iran.

He looked permanently perturbed with Obama’s answers. But he also offered his guest a few compliments. So Obama was correct to oppose the Iraq war as the “wrong battlefield,” O’Reilly observed, but wrong to oppose the surge. (Don’t even bother trying to square that.) Obama gave a good speech in Denver, O’Reilly conceded, but he still “bloviated” about Pakistan.

Obama was cool and cogent throughout, but O’Reilly’s audience presents demographic kryptonite to his political superpowers. Fox News viewers went for Bush by 9:1 last cycle, and prime-time shows — like The Factor — offer an even more conservative spin than daytime programming. Meanwhile, the average age of an O’Reilly viewer is 71. Obama has trouble with older Americans in general, but older Americans who voted for President George W. Bush and tune into O’Reilly? Even they may not be moved by this solid performance:

Fox News is promoting Obama's Thursday appearance.

As Sen. John McCain accepts the GOP presidential nomination on Thursday night, his opponent will enter the “No Spin Zone.”

Yes, Sen. Barack Obama is headed for Fox News, in a bid for Republican votes, by appearing for the first time ever on Bill O’Reilly’s prime-time show. While McCain aims his acceptance speech at voters beyond his base, Obama will be talking to the bright red Republicans who watch O’Reilly’s highly rated show. In the last presidential election, a whopping 88 percent of Fox viewers voted Republican, while only 7 percent voted Democratic.

Obama’s appearance also caps an unusually long and rocky exchange with O’Reilly.

A senior aide to Obama said the controversial anchor “shoved” him during a February altercation in New Hampshire, when O’Reilly tried to confront the Illinois senator to demand he appear on the Fox program. “I’ve worked two [presidential] campaigns, and I’ve never had a member of the press lay hands on a staff member before,” said the aide, Marvin Nicholson, who travels with Obama. Then, at a secret June meeting with Rupert Murdoch, the owner of Fox, and Roger Ailes, who runs it, Obama aired his concerns about the network’s biased coverage and agreed to appear on O’Reilly’s show, according to new reports from Vanity Fair and The Washington Post.

Beyond O’Reilly’s show, Fox News has been under siege this election season by several liberal groups supporting Obama. The channel was shut out of Democratic primary debates, after Democratic activists and bloggers scuttled its attempt to host debates with the Congressional Black Caucus. More recently, Fox was rocked by allegations of anti-Obama bias and racial prejudice in campaigns by Color of Change, a netroots group focused on black issues and civil rights — that was the battle featuring the rapper Nas on the Colbert Report — and a viral video campaign by Brave New Films.

Obama’s exclusive appearance, however, shows that he’s willing to tap the Fox News for potential conservative voters — even as his supporters try to marginalize its political role.