A six-match unbeaten run which was threatening to drag Colchester well clear of the relegation battle came to a shuddering halt at rain-battered Boundary Park.
Early goals by John Eyre and veteran David Eyres were more than enough to give Oldham only their second home win in nine League matches and keep the Latics firmly on course for an automatic promotion spot.
This was a first defeat for Colchester since former Reading stalwart Phil Parkinson took over as manager and the new boss was disappointed with his players' performance.
He admitted: "We got what we deserved in the first half because we didn't keep the ball well enough or have any control of possession.
"The second half was better and we at least came off with some pride but the lads need to win more battles all over the pitch away from home." The troubles did not end there for Parkinson as striker Gareth Williams - a big hit since signing on loan from Crystal Palace - limped off after 26 minutes while goalkeeper Simon Brown stayed in the dressing room at half time.
But delighted home manager Iain Dowie praised his team for their response to a midweek defeat at promotion rivals Bristol City.
"You never want to lose two on the trot and we never looked like doing it after a fantastic first half," he said.
"We absolutely murdered Colchester and don't forget they came here as one of the form teams in our League." It was smooth sailing for Oldham once they took the lead through Eyre in the 13th minute, the midfield man's second goal of the campaign.
A mis-cued shot ran to Eyre well outside the far post but he beat the angle brilliantly with an unstoppable shot from ten yards.
The lead was doubled six minutes later as Paul Murray crossed from the right as Eyres, who turned 39 last month, showed he still has magic in his left foot.
The golden oldie's half-volley from just inside the box flew into the top corner for his 11th strike this term.
The game was all but over but Colchester suffered another blow when Alan White saw red in the 45th minute for impeding Clyde Wijnhard as he raced clear on goal.
The second half was a non-event apart from one shot in the 69th minute by Carl Duguid which hit the bar.