Donaldson struck early and late at Boundary Park, handing the Bees' promotion drive another boost and taking his personal tally to 17 goals this season.
The ex-Crewe and Hibernian frontman gave his side a fifth-minute lead when he met Harry Forrester's left-wing cross with a powerful header from eight yards.
Marcelo Trotta wasted a chance to double Brentford's advantage before Donaldson pounced again with eight minutes to go, slotting in from 12 yards after a pass from Sam Saunders.
Rosler said: "Clayton is one of the best in this division. There are a few good strikers around, but he has been very consistent for the last 18 months in what he contributes to the team.
"His work rate is phenomenal and he can play several positions up front. This season he has been clinical and I'm sure he understands that he has to work hard if he wants to keep this run going.
"His early goal was obviously important, although we played with the handbrake on in the first half and Oldham had more of the ball.
"They had momentum without creating big chances, but after we put Jonathan Douglas into the holding midfield position we were the side with more possession.
"This was a tricky game. Oldham blow hot and cold and they have lost some home games, but they beat Forest last week.
"The longer the game went on, the more you could see our quality. But we shouldn't be looking at the league table - we have to concentrate on performances."