Porter fired a goal in each half to set his seasonal tally to 16 goals with Will Haining also on the mark as Oldham claimed a double over United and gained revenge for their Carling Cup exit at Millmoor earlier in the season.
The Latics, who had secured six wins from their last eight league matches, dominated after a slow start and looked every inch a promotion chasing outfit in the first half. But it was the Millers who had the game's first real chance.
Top scorer Will Hoskins blew a gift-wrapped opportunity following a cracking cross from Ritchie Partridge, his control badly letting him down when through on goal with Les Pogliacomi making a brave block.
A mix up between Ian Sharps and Neil Cutler then let in top scorer Porter but he hooked his shot over.
But the beanpole striker fared better on 28 minutes, heading home the visitors' opener after Leeds target Chris Taylor sent over a juicy cross.
Former Oldham striker Delroy Facey spurned a golden chance to level almost immediately when sent clear, but shot straight at Pogliacomi.
Taylor then saw his downward volley hit over acrobatically by Cutler, but from the resulting corner, the Latics doubled their tally on 41 minutes. Andy Liddell's floated centre was headed emphatically home by Haining, who celebrated his return to the side with his second goal of the campaign.
No doubt handed a roasting by boss Alan Knill at the break, the Millers started the second half in positive fashion after belatedly waking from their slumber.
They grabbed a much-needed lifeline on 54 minutes, Eugen Bopp firing home his second Christmas cracker in three days with a fine downward volley.
The Millers lay siege in a dominant spell which rocked Oldham on their heels, but could not find a leveller.
Porter restored the visitors' two-goal cushion on 67 minutes, latching onto Neal Eardley's fine through ball before coolly slotting home.
United, unbeaten at home since the end of September, pulled one back from the spot on 80 minutes after Simon Charlton's hand ball with Hoskins sending Pogliacomi the wrong way but it was to no avail.