Even fourth place this season, with the promise of better still in the years ahead, defies the reasoned consensus of supporters and pundits alike when George Graham left for Tottenham and, after preferred candidates dropped out of the equation, O'Leary was eventually installed as manager.
"Yes," he smiled, basking, however fleetingly, in the afterglow of Leeds' sixth consecutive league win. "A lot of people told me to get out because the club had gone as far as it could. They said I should learn my trade in the Third Division.
"But I had worked here for two years. I knew what we'd got here. I knew the players, that they could only improve, I felt I could put my hallmark on it."
This ramshackle Derby side were in no state to gauge the authenticity of O'Leary's product, but Leeds' emergence as a threat to Chelsea's position among the continental elite is measure enough of a remarkable development in our game.
O'Leary, the man they reckoned would be too nice, too naive, too gullible to shape a successful Premiership club, has not only taken on Graham's team another stage, he has also stepped clear of his mentor's shadow.
And now he looks further, to the day when he can eclipse Alex Ferguson and Arsene Wenger. That is why he refuses to stand still, to wallow for long in the glory of his initial impact.
In his restlessness to maintain the momentum, he plays down his squad's aspirations because he feels he needs more players to take on Manchester United and Arsenal, and he will hammer home the message until he has what he wants.
O'Leary has come this far largely without the assistance of David Batty, who missed Saturday's match because of flu, and it is difficult to comprehend why the Irishman should have sought such a one-dimensional player when he is blessed with an array of effervescent, flexible talents. They play with joy and bewildering movement.
Derby were simply overwhelmed, despite being presented with an early lead, Lucas Radebe inexplicably taking Vassilis Borbokis's legs from beneath him. No arguments about Mike Reed's decision this time and Francesco Baiano converted the penalty.
Jimmy Floyd Hasselbaink might have given Leeds the advantage before making his decisive contributions to proceedings. The Dutchman supplied the ammunition for Lee Bowyer and Willem Korsten to score and dispatched one of his own with characteristic conviction.
It appeared only boredom on Leeds' part averted an annihilation. They simply toyed with the wretched, wounded creature at their mercy, delaying the final blow until five minutes from the end, an extravagant 25-yard effort from Ian Harte.
Derby contributed to their own demise with strangely ineffectual defending. Horacio Carbonari and Spencer Prior had a torrid afternoon and an injury to Igor Stimac compounded the torment.
"We simply couldn't cope with having so many players out," Smith said. "We got a goal start but couldn't hold on to the lead. In the end they just overran us."
Manchester United and Arsenal still have to visit Elland Road, while Stamford Bridge is on Leeds' travel schedule. All are potentially titanic encounters, and genuine barometers of O'Leary's progress.
"We'll give anybody a game," O'Leary concluded confidently. "United here will be a good game, and they know they will be in for a game here."
Goals: Baiano (pen 4) 0-1; Bowyer (17) 1-1; Hasselbaink (31) 2-1; Korsten (44) 3-1; Harte (85) 4-1.
Leeds United (4-4-2): Martyn; Haaland, Woodgate, Radebe, Harte; Bowyer, Hopkin, McPhail, Korsten (Jones, 88); Hasselbaink, Kewell. Substitutes not used: Wetherall, Ribeiro, Halle, Robinson (gk).
Derby County (3-4-1-2-1): Poom; Stimac (Elliott, 40), Carbonari, Prior; Laursen, Bohinen (Launders, 78), Powell, Borbokis; Baiano (Christie, h-t); Harper, Burton. Substitutes not used: Hoult (gk), Robinson.
Referee: M Reed (Birmingham). Bookings: Leeds: Haaland, Hopkin. Derby: Baiano, Prior.
Man of the match: Hasselbaink.