Tykes boss Andy Ritchie came back to haunt the club where he is a legend as the visitors proved far too good for an out-of-form home team.
Oldham's second home defeat in a week was assured once winger Chris Shuker broke the deadlock on 49 minutes.
Stephen McPhail and Martin Devaney followed up with further goals as Oldham went to pieces in defence and they proved unable to trouble the keeper at the other end.
Oldham, who kicked off in seventh spot and were only behind Barnsley on goal difference, played their best football in the early stages.
Luke Beckett was inches away from reaching a right-wing cross by strike partner Paul Warne and captain Chris Swailes hit a shot on the turn which flew narrowly off target.
But Barnsley striker Marc Richards, who once had a loan spell with Oldham in his younger days as a Blackburn player, showed the visitors would be a threat when he headed over.
Devaney's drive from ten yards was saved by Oldham keeper Lee Grant and the winger also blazed off target.
Oldham's Richard Butcher saw his long-range volley kept out by Nick Colgan but Shuker fired a great chance against the post after 36 minutes.
Shuker's opener arrived four minutes after the break as he pounced on a loose ball ten yards out and drilled a low shot inside Grant's left-hand post.
Paul Hayes missed by inches with a glancing header and Barnsley doubled their lead on 62 minutes as Hayes did well down the left wing and found McPhail in the six-yard box for a simple tap-in.
Oldham had plenty of possession but created little up front, with Beckett and Warne badly off target when they gained a sight of goal.
Barnsley rounded off their victory 11 minutes from time as the outstanding Devaney cut inside from the left and used his right foot to drive a shot into the bottom corner from 16 yards.