Oldham took the lead on 12 minutes before Bulgarian trialist Vladislav Mirchev equalised for the Lilywhites on 15 minutes.
Oldham were reduced to ten men on 63 minutes before Nicholson tapped in the winner in injury time.
Preston remain in second place in the totesport.com league with 15 points from 8 games, 4 points behind leaders Burnley who have played one game less