A subsurface safety valve for use in wells, preferably adapted for placement in a tubing string, having a lock open mechanism for temporarily maintaining the safety valve in the open to flow position. The safety valve includes a closure device, such as a rotatable ball, located in the flow passage of the valve and is opened and closed by a biased operator tube. The operator tube is movable in response to hydraulic fluid conducted to the safety valve operating on a piston carried on the operator tube. A shiftable sleeve and collet is positioned above the operator tube and has a profile for receiving a shifting tool. When shifted, the collet and sleeve act on the operator tube to hold the valve in the open to flow position. The collet engages a recess on the bore wall of the safety valve upon shifting, and the sleeve moves behind the engaged collet head to prevent unlocking. The safety valve can be returned to operation by shifting the sleeve and collet to their original, normal position.