Oldham continued their impressive run of form, making it four wins in five games with victory at Colchester.
French centre-back Jean-Yves M'voto powerfully headed the visitors in front from Lee Croft's corner.
The Latics made the game safe when Matt Derbyshire's shot was initially blocked, and then turned in by unfortunate U's defender Brian Wilson.
Jabo Ibehre missed two chances for Colchester, who have now lost six of their last seven in all competitions.
VIEW FROM THE DRESSING ROOM
Colchester United manager Joe Dunne told BBC Essex:
"Not good enough. Lacklustre. Very disappointing considering we've had two weeks where we haven't had a game. We didn't deserve to win.
"We just didn't play well enough, didn't have enough balls to get on the ball. I don't think we had any passing movements.
"The first goal is a player not doing his job for the club at set-pieces. The second goal is a freak goal, it's going to be shown over and over and we're going to be laughed at.
"We're in a dog fight now. It takes a certain mentality and character to get out of this position, and we need to find it quickly."
Oldham Athletic manager Paul Dickov told BBC Radio Manchester:
"It was the manner of the performance that pleased me. I thought some of the attacking football we played today was top class.
"We had a 10-minute spell after we scored the goal when we stopped doing the right things. They possibly could have got back in it at that time.
"But defensively we looked really solid and, at times, the front four were unplayable."