Ronnie Moore sealed back to back promotions with the Millers during an eight-year stay which made him a club legend, but Rotherham showed no sentiment as they completed a second triumph of the Bank Holiday weekend.
Deon Burton hit the only goal, making it four in three days for the Jamaican international striker.
Rotherham had the better chances and could have sewn up the points earlier if Oldham keeper Chris Day had not been in superb form.
Day made a save from Burton when the Rotherham man jumped for a corner at the far post on just seven minutes.
Oldham made a lively start and striker Chris Killen, who was playing his first game of the season after a hamstring injury, headed over the bar.
Killen also latched on to a cross by Terrell Forbes to draw a reflex save from Neil Cutler, but Rotherham had a clear opening from Martin Butler who sliced off target from 12 yards.
Butler's long-range shot saw Day make a diving stop before the keeper stretched into his top corner to prevent John Mullin scoring with a looping header.
Oldham's Andy Liddell tested Cutler after 50 minutes but Day was there again when Burton found space deep inside the home team's box.
Rotherham were forced to take off Michael Keane after a heavy tackle from Stefan Stam, who himself had gone on early in the game following injury to former Millers centre-back Guy Branston.
It did not upset their rhythm and Mullen saw his shot from ten yards brilliantly kept out by Day.
The winner came ten minutes from time and only seconds after Oldham midfielder Richard Butcher had blasted over the bar from a corner.
Burton ran on to a long ball and as Oldham appealed in vain for handball, he drove his shot high into the net from 14 yards.