The Latics dominated a bruising encounter and sealed the points with goals from Oumare Tounkara and Philipe Morais.
They twice went close to an early breakthrough as Tounkara prodded off target and Warren Feeney headed over from close range.
Tounkara also blasted a good chance into the crowd but the big Frenchman made amends on 40 minutes as he raced on to Chris Taylor’s through-ball and calmly slotted past Brentford keeper Richard Lee.
Owain Tudur Jones wasted a rare Brentford chance after 68 minutes but Oldham soon doubled their lead as Morais cut inside onto his right foot and drilled home a low shot from 12 yards.
Brentford grabbed a late consolation when substitute Robbie Simpson turned eight yards out and fired in a clinical volley.