The Edinburgh-born former Gretna and Livingston man joined the Latics in May 2008 after agreeing a three-year deal with the Boundary Park outfit, but has agreed a move back to Scotland ahead of the new season.
Oldham manager Dave Penney told the club's official website: "I called Greg in and told him Dunfermline had made an approach for him and left the option open to him - stay here and fight for your
place with Dean (Brill) or go back to Scotland for 12 months.
"Greg needs to play games at this stage of his career and following a chat with his family he said yes to the move.
"Hopefully Greg will come back from another season in Scotland with games under his belt and be a better goalkeeper for it."