John Sheridan's side made it four wins from the last five games to make sure a disappointing start to the season was banished further into the past.
Stefan Stam and Paul Warne grabbed the goals as Rotherham suffered a blow in their attempts to overcome the pre-season deduction of ten points for going into administration.
In between, Will Hoskins bagged his seventh of the season for the Millers but they could not force a second equaliser past Oldham's defiant goalkeeper Les Pogliacomi.
Oldham were fresh from a 4-1 beating of Gillingham on Saturday but this was a closer contest which saw Rotherham create enough chances to leave with a point.
Afterwards, visiting manager Alan Knill was left to reflect on two more goals conceded from set pieces by his team.
Oldham were on top during the first half as Chris Taylor, who was standing in at left-back for the injured Simon Charlton, blasted wide at the end of a weaving run across the pitch.
In-form striker Chris Porter only just failed to convert a cross from Neal Eardley, but Rotherham posed their first threat through midfield man Martin Woods who dragged a shot wide of the far post.
Oldham went in front six minutes after the half-time break when Andy Liddell sent over a corner from the right and Dutch defender Stam jumped high above the visiting defence to head beyond helpless goalkeeper Neil Cutler from six yards.
Rotherham's Hoskins nearly equalised within five minutes as he seized on a loose ball deep inside the Oldham penalty area and forced a good save from Pogliacomi.
Hoskins went close again shortly after but the home side were inches from doubling their lead when Warne unleashed a fierce drive from well outside the box and watched it slam against the woodwork.
Rotherham equalised in the 68th minute as Hoskins got on the end of a quick move through the middle and finished well from 12 yards as the keeper dashed off his line.
Oldham showed the confidence of a winning team when they went back in front on 76 minutes from a free-kick by Liddell.
Cutler made a good save from Will Haining but Warne was on hand to slot in the rebound from close range.
Rotherham pressed for a second only to find Pogliacomi doing superbly to thwart Lee Williamson.