The application of an optical klystron configuration to waveguide free electron laser systems is proposed either as a gain enhancement scheme or as a way to reduce the beam current necessary for a given gain requirement. An analytical formulation is developed for these devices in the small signal regime. The feasibility of an optical klystron arrangement for FEL systems operating in the far infrared and submillimetre regions is studied. It is shown that with the excellent beam quality of electrostatic accelerators, the optical klystron scheme is an effective means to improve the small signal gain.