Inheriting The Tonight Show throne this week, Conan O'Brien has taken NBC's late night institution old school.

Steve Allen old.

O'Brien's approach is the most off-the-wall and unpredictable since Steverino invented the job in the '50s.

The impish Irishman favors a spontaneous style. Though not as strong in his opening monologue, he may be the best improviser in Tonight's distinguished line of succession. He also leans heavily on taped bits - doctored news clips and set-up sketches.

Maybe too heavily. His inaugural show was particularly indulgent in its reliance on prerecorded elements. Did he really need the long segment which found him driving around Los Angeles in a 1992 Taurus, set to the music of Billy Ocean?

Of course, O'Brien has had three months to prepare for his new gig, so it's understandable that he has a sizable inventory of prepared material.

There's no question he hit the ground running - with an opening skit on Monday that had him legging it at top speed from Manhattan to Los Angeles. His route took him through the outfield at Wrigley Field in Chicago where he was chased by the grounds crew.

Coincidentally, Jimmy Fallon sprints down the street each night at the top of The Late Show (which follows Tonight). Maybe NBC has its late night personalities on a fitness program.

Even without the aerobics, O'Brien is notably light on his feet, marveling during his Tuesday monologue that he had made it to a second night. "Anything that runs this long on NBC is considered a smash hit," he said.

His first guest, Will Ferrell, had struck a similarly skeptical chord the previous night about O'Brien's chances of longevity, saying "Don't get me wrong. I'm pulling for you, man. But this whole thing's a crapshoot at best."

Among the changes in the O'Brien format is far more banter with his announcer, Andy Richter who stands behind a lectern stage right. For all we saw of or heard from Jay Leno's announcer, John Melendez, he might as well have been in a cave in Pakistan.

Instead, Leno used Kevin Eubanks, his bandleader, as a foil. That seems unlikely to happen with Max Weinberg, the taciturn new musical director.

While Leno saw himself as the people's host, shaking hands with the crowd lining the stage each night like a politician working a rope line, O'Brien prefers to work at an ironic distance. But he's also more animated and expressive than his predecessor, mugging his way through his routines.

The new guy with the pompadored Jimmy Neutron hair, is, Carson-like, establishing some signature gestures right off the bat, like his Chippendale gunslinger pantomime as he comes out for the monologue.

O'Brien peppers his jokes with pop references. Usually they're spot on, like his claim that the tour guides at Universal Studios, the site of his production facility, are always confusing him with actress Tilda Swinton.

Once in a while, he's off the mark. In the middle of a story about shooting the film Angels and Demons, Tom Hanks, the Tuesday guest, hooked his thumbs into his armpits and began flapping his arms like a bird.

"What are you? Chico Marx?" said the host. Umm, you probably meant Red Skelton there, Conan.

Maybe he simply didn't want to name check anyone named Red. In his short run thus far, O'Brien has already acquired the unfortunate nickname, Coco, a play on his first name.

The thing about O'Brien is that he's very hit-or-miss. But at least he's emptying his gun every night.

In its first week, the show has acquired a more expansive, scattershot tone. It feels less rehearsed than Leno's regime, but at times, out of control. For all his free-form irreverence, the new host has come across jittery and overamped in the early going.

O'Brien has managed to turn up the heat on Tonight while dialing back the pace.

Good luck with that, Coco.

Contact staff writer David Hiltbrand at 215-854-4552 or dhiltbrand@phillynews.com. Read his recent work at http://go.philly.com/daveondemand.