The valve operator assembly is provided includes a housing, an input member rotatably mounted, and a transmission mechanism including a translating element and a rotating element connected to the input member, said transmission mechanism being adapted to convert applied rotation of the input member into axial translation of said translating element.The assembly further includes a locking mechanism provided with a casing mounted on the transmission mechanism and/or the housing, with a freewheel including a plurality of locking members mounted in radial contact with the transmission mechanism and with the casing, and with friction means axially disposed between the casing and the housing and mounted in axial contact with said housing. The friction means, the freewheel and the casing cooperate together to lock the transmission mechanism with the housing in a static position of said assembly.