O'Shea sealed the win for the Roar - their first in five games - with a 90th minute penalty at Allianz Stadium on Sunday in the final game of the A-League's inaugural Unite Round.
Dane Ingham's handball gave O'Shea the opportunity to convert and he calmly slotted it home for a win which moves the Roar to eighth spot on the A-League Men table.
The fightback the Roar showed would have heartened Cahn, while Newcastle's latest capitulation will have left Jets boss Rob Stanton frustrated.
Newcastle began in imposing fashion when Dylan Stynes was able to slip a well-weighted pass to Stamatelopoulos and the striker buried the ball past Macklin Freke in the 17th minute.
The Brisbane No.1 was all that was keeping his side in the game, parrying a Trent Buhagiar volley wide after the Jets forward had a goal chalked off for offside.
Brisbane rarely ventured forward but one of their few forays into Newcastle territory nearly resulted in a penalty when Kosta Grozos clattered into Florin Berengeur inside the Jets' box.
Referee Jack Morgan pointed to the spot immediately but after having a second look on VAR overturned his own decision.
Berengeur hobbled off soon afterwards and Newcastle went so close to adding a second.
Grozos rattled the post just before halftime and Buhagiar had an effort bounce clear and over the bar.
But their failure to convert when it mattered came back to bite them, O'Shea going on a jinking run and drawing the Roar level with a low-driven shot in the 62nd minute.
Newcastle hit back almost instantly when Freke fumbled a corner and Phillip Cancar bundled the ball into the Roar net.
But each time the Roar went forward Newcastle looked nervous, and from a corner Jonas Markovski was able to prod the ball over the line to set up a grandstand finish.
When Ingham was penalised for handball, O'Shea was the man to step up and he dispatched the ball past Jets No.1 Ryan Scott to seal the victory.
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