The Bees inflicted Oldham's fourth straight home defeat and suffered few real alarms after Donaldson's early breakthrough.
Oldham's Matt Smith had an effort saved, but Brentford led after just five minutes when Harry Forrester crossed from the left and Donaldson powered home an eight-yard header.
Oldham almost found a quick equaliser, but Smith, Jose Baxter and Cliff Byrne were all narrowly off target.
Donaldson could have doubled the Bees' lead, only to head straight at keeper Dean Bouzanis from close range.
Carl Winchester missed a chance for the Latics, but they had an escape when Marcelo Trotta blazed over for Brentford.
The visitors made sure of victory eight minutes from time though as substitute Sam Saunders played in Donaldson, who calmly slotted past Bouzanis from 12 yards for his 17th goal of the season.
