The Lilywhites struck early on when Jack King slammed home from eight yards to the delight of the home crowd.
The Latics had a chance to level on the 18-minute mark when Jonson Clarke-Harris was fed through by Jonathan Forte only to see his effort deflected away for a corner.
Clarke-Harris seemed most likely to get the afternoon's second goal but was forced into fruitless shots from distance in what became a scrappy affair in the second half.
Garner found himself spurning a chance of his own in the 76th minute, heading wide after an Alan Browne left-wing cross.
The win for Preston sees them to third, just one point behind the automatic promotion places, while Oldham remain in the relegation zone.