Fittingly it was Emile Heskey, the latest recruit in O'Neill's transformation of Villa into a top-flight behemoth, who proved the difference.
His first-half goal, a textbook demonstration of his poise and power as he drove in Gabriel Agbonlahor's flick-on, sustained the Midlands club's remarkable tilt at the title. But the manager could hardly have dared expect such an instant return from his signing, the striker having scored just three times for Wigan this season.
O'Neill was pleasantly taken aback by Heskey's instant impact. 'I thought it would take some time,' he said. 'He was very strong and scored a great goal for us. I'm hoping it will be the first of many.'
As a ninth win on the road put Villa level on points with Liverpool, he could not deny that he was savouring the novelty. 'I'm absolutely thrilled, I wouldn't want to play it down. If nothing else, we're keeping the pressure on.
'We're making it interesting.
I still believe that the other teams up there, Arsenal included, have a bit more about them. It's something new to us, but exciting. We might as well enjoy it while it's there.'
Heskey was never less than full value for his £3.5 million, his foraging runs and intelligent positioning the perfect complements to Agbonlahor's raw pace.
Portsmouth were remorseless in their response, winning 17 corners, but ultimately toothless as this result drew them deeper into the relegation mire.
Manager Tony Adams had little for which to thank Nadir Belhadj after the winger was sent off for dissent, throwing the ball away when referee Peter Walton failed to spot a handball by Carlos Cuellar, although the Algerian should already have been dismissed for an ugly first-half slide on Craig Gardner.
'I thought we were a little unlucky,' Adams said, his side seemingly untroubled after their FA Cup humiliation by Swansea. 'We're creative enough, very organised, but everything is costing us. We make one mistake and we're punished.'
To revive Portsmouth, Adams must rely on the acquisition of Javier Saviola, with the Argentine's signing expected to be completed next week.
There were talismen for Villa all over the pitch, James Milner's creativity in midfield ensuring that Heskey did not want for service. Immediately the forward showed his energy with a succession of swift breaks. Sol Campbell and Sylvain Distin, normally such steadfast centre-backs, were under intense pressure.
Unleashed on the right, loan signing Jermaine Pennant saw one of his piercing low crosses hacked clear but only as far as Portsmouth debutant Hayden Mullins, whose powerful 20-yard drive was deflected narrowly wide off Cuellar. Villa had survived the early surge and earned their reward through Heskey.
Brad Friedel provided the impetus with a clearance headed on by Agbonlahor straight into Heskey's path, the striker's muscular control and silky right-footed finish beyond David James doing the rest.
The confidence coursed through Villa, and it was all Portsmouth could do to prevent them extending their lead. One fearsome Milner strike cannoned off Campbell, emboldening Heskey to move in for another shot on goal. But Adams' side remained undeterred, and Mullins espied a clear chance for his first goal when Pennant's pull-back fell to him, but he contrived only an ugly slice over the bar.
Portsmouth risked paying for their wastefulness as Agbonlahor tore down the right flank, beating Distin with ease but dragging his final ball wide of James' near post.
Portsmouth, compelled to chase the game, were relentless and the atmosphere reached fever pitch when referee Peter Walton dismissed Belhadj. By the end Villa were doing little more than absorbing Portsmouth's attacks. O'Neill was a dervish in the dug-out as his players clung on.