Nicky Wroe's 11th goal of the season, from the penalty spot, and Will Hayhurst's goal at the death took North End's unbeaten home run to six, while Oldham remain above the drop zone on goal difference over Scunthorpe.
The Latics went closest early on as Bobby Reid's free-kick cannoned inches wide off Jose Baxter.
In an increasingly tight game, Baxter then fizzed a 20-yard shot past the upright after 35 minutes.
And five minutes later, Lilywhites goalkeeper Declan Rudd produced a superb stop to keep out the 21-year-old's free-kick.
Shortly after the restart, Preston striker Jack King - who netted his first goal of 2013 on Saturday - crashed a header against the crossbar as the hosts built some momentum.
The home side broke through in the 73rd minute when midfielder Wroe blasted home a penalty after left-back Jonathan Grounds was adjudged to have handled in the area.
And Hayhurst struck into an empty net in the dying minutes after Latics goalkeeper Dean Bouzanis went up for a corner.