The home side were on course for a well-deserved point, but Syers pounced in injury time to give Rovers their first victory in five league and cup games.
Doncaster scored after just three minutes when David Cotterill's left-wing free-kick was touched against the post by Billy Paynter and Rob Jones headed home the rebound from eight yards.
Oldham responded well, but Matt Smith's diving header flashed wide and Jose Baxter's long-range effort was saved by Rovers goalkeeper Gary Woods.
The equaliser came after 27 minutes as French midfielder Youssouff M'Changama let fly from 25 yards and saw his rising shot go through Woods' grasp.
Woods then tipped away Jean Yves Mvoto's thumping header before Oldham goalkeeper Dean Bouzanis thwarted Paynter after a goalmouth scramble.
The sides were closely-matched in a tight second half and Oldham threatened a winner when Smith headed straight at the goalkeeper.
But Rovers snatched victory when Cotterill's cross to the far post was headed in from eight yards by Syers.