A subsea powered override tool is described, for effective and permanent reinstatement of subsea tree valve actuation functionality. The override tool is powered from the Subsea Control Module on the subsea Christmas Tree. The override tool assembly comprise cylinder body (11), piston (8) and cylinder cap (2). A locking collar (1) that is part of the override tool is used to attach the tool to the actuator body on the subsea Tree. The override tool is designed for permanent installation and can lie dormant for extensive periods of time before use.