Second-half goals from Timothee Dieng and William Gros saw Oldham battle from behind to ensure that Newcastle's pre-season preparations got under way in disappointing fashion.
However, Newcastle manager Alan Pardew was able to hand debuts to recent signings Jack Colback, Ayoze Perez and De Jong, with the latter handed the captain's armband.
Former Ajax skipper De Jong opened the scoring after just 14 minutes having struck a free-kick into the Oldham wall.
The Netherlands international pounced on the rebound to lash home an effort from outside the area and offer a glimpse of what Newcastle fans can come to expect from the playmaker.
Tasked with helping fill a creative void following Yohan Cabaye's exit in January, De Jong almost had a second when he came close to connecting with Rolando Aarons' delivery.
While Oldham improved from then on, with the help of a lively Jonathan Forte, they trailed at the break.
Perez was one of a host of substitutes introduced early in the second half, shortly before the hosts levelled.
It was another debutant on target for Oldham as Dieng headed home a David Mellor corner past Jak Alnwick.
Substitute Gros then got in on the act as his shot from the edge of the area slipped through Alnwick's grasp with 12 minutes remaining to hand the League One side victory.
Pardew's men will jet out to New Zealand to continue their pre-season preparations, starting with a clash with Sydney FC at the Forsyth Barr Stadium in Dunedin.
