The Latics sensed no danger when he moved onto the ball deep on the right, but they reckoned without the power he was to pack into his 11th goal this season.
The ball never rose more than a few inches but, although Mark Crossley launched into a full-length dive, the shot flew across him and into the far corner.
Rovers had been under pressure from a lively Oldham side powerfully led by Lee Hughes, but simply had to be inspired by Lambert's strike.
He had already drawn a good save from Crossley with a free-kick after Daniel Jones had handled 30 yards out and he went on to claim and assist for Rovers' second goal just after the half-hour.
Rising to Stuart Campbell's fifth corner, he knocked the ball to his left where Chris Lines reacted with a quick thrust for his second goal of the campaign.
Oldham could hardly believe they were going in two down after Hughes and Deane Smalley had exposed a defence lacking two injured central defenders.
Craig Disley passed up a clear-cut chance to add a third before the break and Rovers were in need of that as Hughes led Oldham's fight back.
Steve Phillips did well to grab a Hughes header and then skipper Campbell rescued his side with a goalline clearance.
Rovers were under siege for a long spell but defended strongly to extend their unbeaten run to six games, while for Oldham it was a second league defeat.