The visitors were out-classed - and booed off by their travelling fans - as manager Leroy Rosenior, who is serving a touchline ban, watched helplessly from the stands.
The Bees have now gone ten games without a win and were lucky to escape defeat by a far heavier margin.
Chris Porter missed several chances but the Latics still cruised to the three points.
Oldham went ahead in the 15th minute after a delightful ball into the box from veteran defender Simon Charlton, Brentford keeper Clark Masters came off his line but Paul Warne got there first to blast the ball inside the far post from ten yards.
Masters denied Andy Liddell and scrambled to his feet to somehow deny Porter before winger Chris Taylor blasted a shot off target.
The second goal came just after half-time when Charlton crossed from the left and Gary McDonald blasted in with his right foot from ten yards.
Stefan Stam hit the woodwork before Masters thwarted Porter, and Charlton and Warne also wasted chances.
Brentford only chance came with five minutes left when Gavin Tomlin hit the bar following a poor back-pass by Will Haining.
Richie Wellens completed the Bees' misery in stoppage-time when he burst into the box and charged down an attempted clearance after Masters had saved his initial shot.