LEE JOHNSON made a winning return to Boundary Park just 17 days after leaving the Latics to take charge of Barnsley, who stretched their winning run to six matches to strengthen their play-off charge.
A brace from central defender Lewin Nyatanga and another from Conor Hourihane kept Barnsley in the play-off hunt, while Oldham had player-coach Adam Lockwood dismissed in stoppage time for a second bookable offence for blocking a Reds player.
Sections of the home faithful barracked their former manager, who, however, received top backing from the 3,000-strong Reds following.
Johnson made just one change to the side which beat Walsall 3-0 in their last match.
Cardiff loanee Declan John got international clearance to feature in place of left-back George Smith, who dropped to the bench alongside Jabo Ibehre, who was on loan at Oldham earlier in the season.
The hosts were ravaged by injuries which ruled out eight players and they also had Brian Wilson suspended after two yellow cards in the 2-1 defeat Yeovil.
Connor Brown switched across to his preferred right-back slot and Mat Sadler, who joined on loan from Rotherham United on Friday, made his debut at left-back. Bolton youngster Luke Woodland also came in for his bow in midfield. after signing an emergency youth loan.
It was a quiet opening but Barnsley took the lead in the 32nd minute when Milan Lalkovic failed to make contact with Hourihane’s free-kick but a distracted Jake Kean saw the ball bounce off his legs and Nyatanga hit home the rebound past the Oldham goalkeeper.
It was 2-0 in the 40th minute when Josh Scowen’s inswinging corner caused confusion and Carl Winchester could only blast his clearance against Hourihane and the ball ricocheted into the net.
Oldham grabbed a lifeline before the break when Conor Wilkinson teed up Mike Jones to drive the ball home from just inside the area - it was the first goal the Reds had conceded in six matches.
Hourihane almost made it 3-1 in stoppage time with a shot from outside the area which skimmed the bar.
Oldham pressed after the break but were undone when a far-post header from Hourihane’s free-kick struck the bar and Nyatanga forced home.
Adam Davies made a couple of late saves from Jonathan Forte and Rhys Murphy but the Reds held on comfortably enough.