Most OF the time, Nick Cave does an expert job of persuading us that he's the Devil. When he's not doing that, he's being something even scarier: a cabaret artiste. "Well, thank you. Thank you very much," he smarmed in between songs at the Brixton Academy on Wednesday, with a Mike Flowers-ish grace unusual for someone who has just bellowed a ditty about caving in a girl's head with a rock. But don't be afraid. When the music starts up again, he's the blood-crazed lunatic we know and love.

As rock'n'roll theatre, you can't fault it. Auld Nick acted out each song (look no further for the source of John Travolta's Pulp Fiction character, Vincent Vega: a cool but edgy, disco-dancing, cheap-suited assassin with a greased back stack of long oily hair). He paced the stage restlessly, he pounded the microphone to pieces. Ever the crazy preacher, he roared in his thick, deep voice. And just in case his deranged behaviour didn't give you the gist of what the material was about, the splendid Bad Seeds were on hand with gothic piano chimes and the churchyard clong of the tubular bells. Cave doesn't like his audience to miss the point.

He played only three songs from his Murder Ballads album (Mute), but the rest was in the same punctured vein: a limited genre (indie-country- voodoo-blues) that appeals primarily to back-combed girls who sleep in coffins. On "Mercy" he sang: "My death it always bores me." Why does he go on about it, then? From Cave, you can have any colour as long as it's arterial red.

One Murder Ballad was "Where the Wild Roses Grow", a duet with "Miss" Kylie Minogue. I can appreciate why it might appeal to Cave's sense of humour to corrupt this little sparrow with his vulture's song, and I can understand why a middle-aged man, or indeed any man, would enjoy draping his arm round a slim bimbette; but if she can't hold a tune, he should indulge these fantasies in private. After an ill-rehearsed "O'Malley's Bar", Cave had a long on-stage conference with the Bad Seeds and then decided that they would finish the show there. It was a concert that started with smoke and fire, and ended up running out of steam.

The Knebworth masterplan bit the dust. I had intended to review two of Oasis's support acts, the Prodigy and the Chemical Brothers, but that idea got lost amid a crowd from a Biblical epic. Almost every inch of the yellow-brown grass that wasn't covered with flattened Kia-Ora cartons and crisp packets was covered in people. Not much room to dance, then. And, sober in the mid-afternoon sun, who would want to? Anyone with a basic grasp of gig arithmetic knows that 125,000 people divided by three bars equals two-hour queues.

And so to the headliners. Unlike the bands to whom Knebworth usually plays host, Oasis's music has a punky roughness, but last Saturday's event illustrated blazingly why punk bands sprang up to bite the ankles of the dinosaurs in the first place. If fans spend all day in a queue a mile from the stage, while Oasis are in backstage luxury and even the hangers-on have a free bar - I observed it first-hand, for journalistic purposes - anyone would feel that the distance between band and fans, in many ways, is too great; that the would-be working-class heroes may be sliding into the decadence patented by the Rolling Stones in the 1970s. And it's the Stones, not the Beatles, with whom Oasis should be compared: they're both primal bad-boy rock bands who copy the Fab Four. Oasis are making a similar mark to the Stones, too. For decades to come, garage bands will practise, not just with "Satisfaction", but with "Live Forever" and "Supersonic".

That these songs are already standards is one reason why Oasis's performances are irrelevant beside the music and the atmosphere. All I could see of the group last Saturday was their images on the monitors, and I'm sure several thousand others shared my suspicion that these pictures came from a video filmed at one of the previous weekend's Loch Lomond shows. It was like putting money into a pub jukebox to hear a tune you can listen to for free on your stereo at home.

There were two "new" songs, "It's Getting Better, Man" (where did they get that title?) and "My Big Mouth"; "Whatever" had a swampy harmonica smeared over the neat George Martin-style strings; and "Champagne Supernova" had Oasis's spiritual big brother John Squire joining in on guitar. But it all seemed familiar, as if the band can't keep up with their career's phenomenal velocity. At the show's start, Liam, in his Lennon specs and safari suit, sang a mocking refrain of "Parklife". He's going at such supersonic speed that he can't see who he's racing against anymore. (What's the Story) Morning Glory? (Creation) has now reportedly outsold every Beatles album, and yet Liam is still bothered about Blur.

Noel is aware of what's happening, but isn't sure how to handle it. He no longer keeps his shades on and keeps his cool. He asked the crowd to dance, and on "Don't Look Back in Anger", he revised the lyric, "Please don't put your life in the hand / Of a rock'n'roll band / Who'll throw it all away" so that it said: "They're never gonna throw it away". It was quite touching.

I don't begrudge Oasis their place in the record books. But now they've proved themselves. They've beaten everyone. It's time to concentrate on improving the music and the spectacle, to increase the quality as much as the quantity.