Oldham looked to be heading to their first win under new boss Joe Royle when they led 1-0 at half-time thanks to an excellent goal by Danny Whittaker.
But the visitors saw their outside hopes of the play-offs all but disappear after the break as Scott McGleish levelled with a penalty before Demetriou grabbed the later winner.
Oldham, who had started brightly, almost grabbed the lead when Lewis Alessandra went close with an audacious chip that just cleared the Orient bar.
Little was seen of the home side as an attacking force in the first half as visiting keeper Shane Supple was a virtual spectator.
As Oldham continued to press skipper Sean Gregan forced home goalkeeper Jamie Jones to parry the ball away after he fired in a 20-yard shot, following Whittaker's free-kick.
Oldham gained their reward 11 minutes before the break when Whittaker broke the deadlock with a stunning strike.
He picked up a pass from Chris Taylor and ran 20 yards before unleashing a fearsome shot from the edge of the area that flew past Jones and into the top left-hand corner of the net.
Orient's one and only effort of the first half came just before the break when Paul Terry fired just wide.
O's manager Geraint Williams threw substitute Dean Morgan into the action at the start of the second half and Orient looked a different side.
Morgan twice went close before Orient levelled on the hour, thanks to a contentious penalty award.
Morgan ran into Oldham defender Reuben Hazell, but referee Iain Williamson pointed to the spot and McGleish stepped up to fire the ball home as keeper Supple went the wrong way.
It looked as if Oldham had weathered the storm until Charlie Daniels played in a ball to Demetriou, who sent a 20-yard shot crashing into the net via a post as Orient recorded only their fifth home win of the season.