Gary Madine scored for the home side before the break and Miguel Llera and Ryan Lowe added to the tally in the second half.
Oldham started promisingly and Robbie Simpson headed just over the bar within the first 10 minutes. The hosts then created a few chances of their own and Nile Ranger nearly put them ahead when he took advantage of a misplaced pass but hit his shot straight at Dean Bouzanis.
Madine took advantage of some slack marking to head in Llera's cross from close range in the 43rd minute to open the scoring.
Oldham nearly equalised just after the break when Shefki Kuqi broke away but scuffed hit his right-footed shot and Stephen Bywater saved easily.
Llera made it two when he headed in substitute Keith Treacy's free-kick after Madine had been brought down by Simpson in the 67th minute. With 10 minutes to play substitute Lowe headed in Treacy's cross from within the six-yard box to put the result beyond doubt as the Latics lost for the fifth game running.