The Lilywhites are on the verge of clinching top-flight football after seeing off Down by five points on a greasy Saturday evening in Newry.
O’Neill brought David Slattery off the bench at half-time and the Confey man scored two goals to seal a crucial win in front of 1,700 spectators.
Victory for Down would have taken them level with Kildare at the head of affairs — but Slattery was the game-changer.
Midfielder Kevin Feely was also massive for the winners, winning primary possession repeatedly in a congested middle third, kicking five frees, and providing the assist for Slattery’s opening goal.
At the end of a low-scoring first half, Kildare led by 0-4 to 0-3, having played with a breeze.
But Slattery netted with his first contribution in the 37th minute, after Daniel Flynn and Feely created the opening, before slotting home brilliantly 20 minutes from time from Niall Kelly’s handpass.
Down had a bright start from Barry O’Hagan and a big second half from Caolan Mooney but they finished with 13 players as Darren O’Hagan went off injured and Jerome Johnston was black-carded, after all six permitted substitutions were used.
Kildare boss O’Neill played down what will be described as a masterstroke, when he plumped for Slattery to add impetus to the forward line.
He said: “The squad is there for a reason, it’s nothing to do with the manager or selectors. We always say to the subs — make an impact, make a difference. His difference was phenomenal, to kick two goals.”
Slattery and captain Eoin Doyle went off injured before the finish but O’Neill believes he has increasing strength in depth.
Chris Healy didn’t start with a toe injury, while Ben McCormack and Neil Flynn are also on the injured list.
But Kildare still managed to negotiate a tricky assignment against a Down team that had won its previous two matches.
Down boss Eamonn Burns admitted: “They finished the game stronger. I thought, on the night, they deserved their win. Physically, they’re very strong and ran the ball quite well. We knew that was coming down the line at us. They got the scores at the crucial times and won the game.”
D O’Hanlon 0-4f, C Mooney & B O’Hagan 0-2 each, K McKernan & J Johnston 0-1 each.
D Slattery 2-0, K Feely 0-5f, M Donnellan (45), O Lyons, P Cribbin & T Moolick 0-1 each.
M Cunningham; R McAleenan, B McArdle, D O’Hagan; D O’Hanlon, C McGovern, C Mooney; A Carr, P Turley; K McKernan, C Maginn, J Murphy; S Millar, B O’Hagan, R Johnston.
A Davidson for Johnston (55), N Donnelly for Carr (57), J Johnston for Davidson (b/c 63), M Poland for Maginn (66), S Dornan for Millar (69), C Magee for Turley (69).
M Donnellan; M O’Grady, D Hyland, O Lyons; J Byrne, E Doyle, K Cribbin; K Feely, T Moolick; F Conway, N Kelly, P Cribbin; C McNally, D Flynn, C Hartley.
D Slattery for McNally (h.t.), F Dowling for Hartley (54), É Callaghan for Slattery (65), E Heavey for Moolick (68), S Ryan for Conway (70), L Healy for Doyle (70+5).
N Cullen (Fermanagh).