Forest had two men sent off on a day they will want to forget quickly and the Latics took full advantage by racking up their biggest win of the season.
Sammy Clingan received a straight red card for violent conduct with the score goalless, prompting the home side to grab total control and a 3-0 lead by half-time.
And after Danny Cullip was sent off midway through the second half, Oldham completed the rout to make it four wins out of four over the holiday period.
Oldham were already on top before Clingan got his marching orders and almost took the lead on ten minutes, when Andy Liddell saw his 20-yard drive tipped away by keeper Paul Smith.
Clingan was sent off ten minutes later after appearing to lead with his elbow as he jumped for a header with Oldham captain Will Haining.
The floodgates opened on the half hour as Liddell delivered a pin-point cross to Paul Warne, who stole in at the near post to head home from six yards.
It was soon two for the Latics as Liddell's volley was beaten away by Smith and top scorer Chris Porter converted the rebound for his 18th goal of the season.
Forest fell apart in defence and Nicky Southall chopped down Chris Taylor on the edge of the box to concede the first of two penalties.
The visitors thought the offence was the other side of the line but Liddell stepped up to beat Smith away to his left.
Oldham took their foot off the gas but they were four up midway through the second half following a burst through the middle by Porter and a professional foul from centre-half Cullip.
Referee Kevin Friend had no choice but to dismiss Cullip as Porter was clean through. This time Liddell sent Smith the wrong way.
Forest's only sight of goal came in the last minute when striker Nathan Tyson, who had been dropped from the starting line-up, forced a good save from Les Pogliacomi after running 40 yards.
But Oldham inevitably had the last word as a Liddell drive was saved and substitute Craig Rocastle slotted in the loose ball from 12 yards.