A stunning goal from Lee Frecklington put the seal on more away success for the Millers and extended their unbeaten run to 11 matches.
Defender Mark Bradley, with his second goal of the season, set them on their way in a dominant first half to line up the 11th away league win of the season, eight of them in the last 11 away trips.
Frecklington’s super strike, on the run from 25 yards, killed off any hopes of a comeback from Oldham who should have been sunk in the opening half-hour.
Rotherham had early joy at three set-pieces, Arnason twice going close and the other brought Rotherham’s now customary rejected penalty appeal from a strong claim.
Arnason’s long throw was nodded on by Craig Morgan and as Kieran Agard closed in at the far post, he appeared to be pushed over six yards out when he was poised to head in. And, as is also customary, boss Evans was out of the dug-out, arms spread wide, appealing vehemently.
There was a let-off for Rotherham from Oldham’s first danger moment. Ex-Doncaster man Adam Lockwood executed a backheel from a Gary Harkins shot and, fortunately, it went straight into the midriff of Adam Collin.
But the goal Rotherham had threatened arrived on 26 minutes from an unlikely source.
It followed a corner, Richie Smallwood - an effective performer all night - struck an angled 20 yarder that Mark Oxley parried and Bradley struck a controlled half-volley from 10 yards.
Another almost followed immediately, Alex Revell’s header from a corner hitting a post and being cleared off the line.
Then Smallwood sent Ben Pringle away, his centre was perfect for Agard in front of goal but he failed to connect.
Rotherham should have been punished for not being out of sight. Indecision by Joe Skarz let in Charlie MacDonald and Collin saved with his leg.
Then, after Collin pushed away a fierce drive from Harkins, MacDonald should have finished from an angle but fired over an empty net.
Oldham started to have more possession after the break but with Bradley, helped by James Tavernier, checking big dangerman Harkins, they could not force openings.
Rotherham didn’t have the same sort of openings as they had created earlier before Frecklington’s 64th minute stunner.
Pringle gained possession on the left and switched a ball inside as Frecklington broke forward. He had Tavernier moving outside him and free but the midfielder unleashed a 25 yarder which left Oxley groping as it flashed into the net. It was his 12th goal of the season.
A late save by Collin from MacDonald from the only on-target effort, ensured the clean sheet and yet another happy Tuesday night away trip.
“It was comfortable,” said Evans, who was left purring at Frecklington’s “wonderful” strike. “It should have been sorted in a really dominant first half when we played some outstanding football.”