A goal in each half was enough to secure Rotherham victory over Latics on a cold Tuesday night fixture.
Mark Bradley and Lee Frecklington got the goals to consign Latics to back-to-back home defeats.
It took a long time for the game to get going as both sides enjoyed good spells of possession.
Kieran Agard had a few efforts during the opening half of the first half, winning headers and causing Latics defenders problems.
John-Paul Kissock and Rhys Turner, who were making their home debuts for Latics, were causing a nuisance in attack but unfortunately couldn't get the ball to stick up front.
Rotherham did take the lead on 26 minutes through Bradley. A corner wasn't dealt with and the ball was parried out to the midfielder who struck past Mark Oxley to give the Millers the lead.
The visitors were in the ascendancy and went in search for more. James Wilson cleared well off the line from Alex Revell and James Tavernier fired an effort harmlessly over Oxley's bar.
Latics grew into the game and Charlie MacDonald had two golden chances to bring the home side level. The striker managed to get behind the defence ten minutes before the break but his effort was well saved by Adam Collin in the Rotherham goal.
Minutes before half time, MacDonald was presented with another opportunity to bring Latics level but blasted over the bar. Gary Harkins struck a volley from distance which the goalkeeper could only parry but the striker, who was presented with an open goal, could only thunder his effort over the bar.
The second half started with little opportunities for both sides. Quick thinking from James Wilson allowed James Wesolowski to cross for Turner, but the striker's header was comfortably save.
The Millers extended their lead midway through the second half through Frecklington. After neat play in the midfield, the ball fell for the attacker who struck from 25 yards to give Oxley no chance in Latics goal.
With Rotherham dominating possession, they almost made it three soon after the goal. Tavernier fizzed a ball across goal but fortunately for Latics, no player could get on the end of the cross.
Substitute Danny Philliskirk was presented with a great opportunity to reduce the arrears with ten minutes remaining but the Rotherham goalkeeper saved well from six yards to deny the striker.
Although Latics tried, they couldn't find a way past the Rotherham defence and left Boundary Park without any points.
Oldham Athletic: Oxley, Grounds, Lockwood, Wilson, Mellor, Harkins, Smith, Wesolowski, Kissock (Winchester 59'), MacDonald (Millar 85'), Turner(Philliskirk 58').
Subs Not Used: Brown, Dunfield, Rachubka, Byrnes.
Rotherham United: Collin, Skarz, Arnason, Frecklington (Addison 90'), Revell, Bradley (O'Connor 86'), Pringle (Adams 90'), Morgan, Agard, Tavernier, Smallwood.
Subs Not Used: Shearer, Davis, Vuckic, Hitchcock.
Oldham Athletic 0-2 Rotherham United (Bradley 26', Frecklington 64')
Attendance: 3745 (1119 Visiting Supporters)