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."
Colchester 01 Cousins 04 Okuonghae Booked 18 Eastman 20 Wilson 41 Potts 14 Bond (Gilbey - 70' ) 15 Bean Booked (Wright - 56' ) 22 Wordsworth 11 Sears 12 Ibehre 16 Henderson (Morrison - 79' ) Substitutes 35 Pentney 03 Rose 40 Thompson 08 O'Toole 28 Gilbey 30 Wright 09 Morrison Oldham 01 Cisak Booked 02 Brown Booked (Byrne - 46' ) 03 Grounds 05 Mvoto 23 Wabara 04 Wesolowski 07 Croft (Simpson - 90' ) 08 Furman 09 Derbyshire 11 Montano (M'changama - 90' Booked ) 20 Baxter Substitutes 25 Jacob 16 Byrne 06 Tarkowski 22 M'changama 28 Sutherland 10 Simpson 14 Smith