A systematic procedure is considered for the synthesis of an optimal control law for damping large oscillations associated with transient conditions in power systems. A new approach is presented, which uses the dynamic programming approach of the constant-value control problem. The application of this method to a power-system optimal control problem is illustrated by considering a single machine connected to an infinite bus, taking into account the velocity governor, which is represented by a first-order response. The synthesized controls are then used to damp large oscillations associated with transient conditions. Furthermore, the system responses given by these controls are compared with those obtained by the conventional control, and numerical results are shown.