O'Leary, 24, whose contract runs up to and including the game on January 2 with Cheltenham, becomes McCall's ninth summer signing and will provide cover for central pairing Lee Bullock and Michael Flynn.
The player capped an impressive trial match last week with a goal in the 3-3 draw with Carlisle.
McCall told the Telegraph & Argus: "The player probably wanted a bit longer. But this gives him the chance to come knocking on my door again at Christmas and asking for more if he does well."
Meanwhile, veteran striker Peter Thorne will be handed the captain's armband at Notts County for the season's opener on Saturday.