Chris Porter and Paul Warne both scored on their debuts to bring Yeovil back down to earth after last season's heroics as champions of League Two.
Porter, who signed from neighbouring Bury, and ex-Rotherham man Warne pounced from close range in the space of three first-half minutes.
Yeovil battled gamely to find a way back into their first ever match at this level, but their efforts were repelled by an organised Oldham defence.
Yeovil carved out the first chance when Phil Jevons, last season's top scorer with 29 goals in all competitions, missed the target from 10 yards.
But Oldham, whose 12 summer signings have given Ronnie Moore's squad a completely different look, gained the upper hand.
Porter headed at goalkeeper Stephen Collis and made no mistake with his second opening in the 24th minute.
A long throw by defender Rob Scott brought confusion to the Yeovil defence and Porter stole in to send his header past Collis from six yards.
Richie Wellens, on his Oldham debut after joining in a swap deal from Blackpool, was the man behind the vital second goal.
He cut in from the left and found Warne who rifled into the roof of the net from eight yards.
Yeovil responded with glimpses of goal for Efe Sodje and Darren Way and in the second half they looked more of a match for the home side.
But Sodje headed over again in the 70th minute and Yeovil would have lost by a bigger margin if Warne had been able to capitalise on a glorious pass from Andy Liddell in the closing stages.
Oldham paraded most of their new arrivals and with Ronnie Moore at the helm they should have the experience to avoid a repeat of last season's relegation fight.
Yeovil are renowned for their attractive football but they may need a sharper cutting edge in the games to come.