Myles Weston gave the Londoners the lead after just four minutes when he latched onto a Charlie MacDonald throughball to round keeper Dean Brill and coolly slot home.
But 10 minutes later the visitors were level, Dean Furman sweeping home Oumare Tounkara’s right-wing cross from the edge of the box.
Brentford saw two efforts ruled out for offside before the break but after the interval it was all Oldham.
Within a minute of the restart Chris Taylor was slipped in by Tounkara and fired home past a full stretched Ben Hamer to make it 2-1.
Just after the hour mark Taylor was again at the centre of things when he won his side a penalty which Dale Stephens duly hammered home. The home side rarely looked like getting back into the game as Oldham played keep ball for long spells, heaping more misery on Bees boss Andy Scott.