Leicester mounted a storming finish, but although 33-year-old Tony Cottee poached the 199th League goal of his career with 14 minutes left, the home side could not further penetrate a Leeds defence superbly marshalled by Lucas Radebe - who returned from playing in South Africa only yesterday - and remain without a Premiership win in 1999.
At least, if there was small consolation for the home side, they put behind them the awful drubbing suffered at the hands of Arsenal, a defeat that so perplexed O'Neill that he walked along the Highbury touchline after the final whistle to apologise to the Leicester fans.
Such a gesture was not necessary last night but there was still no happy 48th birthday for O'Neill, whose team's sequence of seven Premiership matches without a win is threatening to undermine their season despite reaching the Worthington Cup final. Now Leicester are beginning to risk being drawn towards a relegation battle.
The concession of an early goal was precisely what was not what the Ulsterman required, but it came as Leeds, who moved up to fourth place and contenders for a Uefa Cup place, adopted a strategy designed to absorb Leicester pressure before stinging them with swift and incisive counter-attacks.
O'Leary's side is not short of swift legs, the most dangerous belonging to 19-year-old Kewell, who gave them the lead by arriving at speed at the near post to sweep home Jimmy Floyd Hasselbaink's low left-wing cross despite the close proximity of several defenders.
But for the wise positioning of Kasey Keller, whom Leicester twice had to thank for sparing further setbacks after both Hasselbaink and Kewell had found themselves one-on-one, Leeds might have tied up the points before half-time. They might have had a penalty too when Willem Korsten got tangled up in Andy Impey's legs.
Heskey, still troubled by a back injury, failed to reappear after half- time, and although a couple of scrambles close to Nigel Martyn's goal might have produced an equaliser, instead Leeds extended their advantage with a goal springing from a lucky break, but deserved none the less. Leicester cleared a throw-in only as far as Korsten, who struck the ball powerfully towards the bottom left-hand corner. Keller covered the shot, but so too did Robert Ullathorne, the ball rebounding off the defender almost directly to the feet of Smith who had only to stroke it into the empty net.
Leicester then threw on Arnar Gunnlaugsson, their pounds 1.5m signing from Bolton, in place of Impey. The Icelander gave Leicester some extra inspiration and with 14 minutes remaining they pulled a goal back when Muzzy Izzet's pass sent Cottee through to clip an angled shot past Martyn.
Leicester City (3-5-2): Keller; Kaamark, Elliott, Ullathorne; Impey (Gunnlaugsson, 65), Savage, Lennon, Izzet, Guppy; Heskey (Marshall, h-t), Cottee. Substitutes not used: Arphexad (gk), Taggart, Zagorakis.
Leeds United (4-4-2): Martyn; Haaland, Wetherall, Radebe, Harte; Kewell, Hopkin, Bowyer, Korsten; Hasselbaink (Ribeiro, 90), Smith. Substitutes not used: Robinson (gk), Wijnhard, Halle, Jones.
Referee: G Barber (Pyford).
Results, page 25