Abarth tackled the Fiat 850 immediately after its introduction. The OT850 / 135 and OT850 / 150 versions of the Berlina were released that used the original 843 cc engine. The name referred to the maximum speed that Abarth managed to squeeze out. The OT 850/135 had the normal chassis, the OT 850/150 that of the Special with disc brakes.
As true Abarth development, the OT-1000 and OT-1600 were delivered with different engines. While the OT 1000 still resembled the OT 850/150 in terms of performance and price, the OT 1600 was something very special and only for the brave (and rich) because one and a half times as fast and 3 times as expensive as an OT 850/135 (and almost 4 times as expensive as a regular 850!)