Colin Calderwood's table-topping side had registered six wins and a draw in their opening seven games but were completely outclassed by the visitors.
Chris Porter struck twice during a first half dominated by Oldham as Forest faltered for the first time.
And, although the home side were more spirited after the break, John Sheridan's side were never really troubled as they claimed their first points of the season away from Boundary Park.
Not for the first time this season goalkeeper Paul Smith had to make several decent saves to maintain parity for Forest in the opening stages.
Forest fans were soon becoming restless and their sense of unease was only increased when the visitors took the lead in the 29th minute.
By that stage Smith had already denied Gary McDonald as he hit a shot on the turn from the edge of the box and had been stretched even further by a Paul Warne drive.
But the Forest number one could do little as McDonald again tested him with a rising shot which he again parried but saw fall invitingly at the feet of Porter who had the simple task of driving the ball home from close range.
Prior to that Forest's chances had been limited with keeper Les Pogliacomi not having to produce a meaningful save.
Forest finally shook off their malaise in the 40th minute as Nicky Southall almost conjured up a spectacular equaliser but Pogliacomi was equal to the task.
But it was Oldham who were to extend their lead in first-half injury-time as Paul Edwards eluded the Forest defence to fire in a high, teasing cross from the left and Porter was again the man in the right place to direct the ball in from close range.
Forest introduced Junior Agogo for Jack Lester at half-time and the substitute had an immediate impact, winning a free kick on the edge of the box from which Southall forced another fine save from Pogliacomi.
But with Oldham happy to hold out for victory, the chances after the interval were few and far between.