Petrasso has linked up with the Latics ahead of their match against Hoops team-mate Michael Harriman's Gillingham on Saturday.
The Canada Under-20 international had been set to join Oldham's Sky Bet League One rivals County - managed by recently-departed Rangers midfielder Shaun Derry - but the move was scrapped.
Petrasso featured 19 times in the QPR Under-18 title success last season and was rewarded with a two-year professional contract.
The 18-year-old has made nine appearances for Marc Bircham's Under-21 side this campaign, scoring six goals.
His loan at Boundary Park will run until December 21.