The duo had more than a five-minute advantage over a big group that contained Mathieu van der Poel, Greg Van Avermaet and Magnus Cort Neilsen with 50 kilometres to go, but due to a lack of co-operation on the circuit around the Italian town, they were unable to bring them back.
Vermeersch and Oss worked well together and kept their advantage to more than two minutes in the final kilometres.
With around seven kilometres left, and with the race to be won by the front two, Vermeersch attacked, gapping Oss on the tight tracks as Van der Poel, Van Avermaet and Alessandro De Marchi chased behind.
Once he had got up the road, the Belgian Alpecin-Fenix rider looked peerless on the gravel, eventually taking the title by 43 seconds over Oss.
Behind, Van der Poel finished third.
