Barnsley had failed to score in four games but rocked the visitors with three goals in six minutes.
They took just eight minutes to get into their stride when Daniel Nardiello claimed his second of the season.
The stylish opener came when Paul Hayes set up Jacob Burns who chipped over a clever ball for Nardiello to head in.
Four minutes later Chris Shuker was the creator for midfielder Martin Devaney who drove a low left-foot shot past Chris Day.
And two minutes later it was Shuker's turn to claim his first of two when he picked up a pass from Hayes and slotted his shot inside the post.
Robbie Williams could have added another but Day held on to his 30-yard free-kick and when Hayes volleyed from close in the ball smashed on to the bar.
Oldham battled back from the hammer blow and went close through Luke Beckett and Guy Branston and a David Eyres corner that deflected on to the post.
Barnsley could have stretched their lead early in the second half but Brian Howard saw his shot well saved by Day who dived full stretch to turn it away.
Then Devaney exploited space on the left and rounded the defence before smashing a rising shot that drifted just wide.
But 10 minutes into the second half another impressive move gave Shuker his second of the afternoon.
Williams played the ball down the left and then a brilliant flick for Nardiello wrong footed Marc Tierney and Shuker produced a cool finish to give the home side their biggest win of the season.
It left angry travelling fans booing off Ronnie Moore and his dejected Oldham side as Barnsley went above them in the table on goal difference.