OSY () (Odikes Sygkoinonies S.A.), a subsidiary company of OASA (Athens urban transport organisation), is the main operator of buses and trolleybuses in Athens. As of 2017, its network consists of about 322 bus lines which span the Athens Metropolitan Area, with a fleet of 2,375 buses buses and trolleybuses. Of those 2,375 buses 619 run on compressed natural gas, making up the largest fleet of natural gas-powered buses in Europe, and 354 are electric buses (trolleybuses). All of the 354 trolleybuses are equipped to enable them to run on diesel in case of power failure.