An analysis is presented of the role which stable electron oscillations of large amplitude may play in determining observed ``steady‐state'' characteristics of a plasma‐filled thermionic diode. The mechanism considered is that of coupling between electrostatic plasma oscillations, driven by the energy of the entrant electron ``beam'' from the emitter surface, and the potential distribution in the emitter sheath region. In operation such that net current flow is controlled by the sheath potential distribution, it is found that the time‐averaged net current may increase markedly in transition from nonoscillatory to oscillatory operation, with a concurrent change in cell and load potential drops such that the diode acts as a negative resistance source of potential over a limited range. Conditions which restrict attainment of the oscillatory mode are discussed, and it is found that large amplitude electron waves may be generated and maintained over a fairly wide, and experimentally accessible range of plasma electron density. Comparison is made with some experimental results, and an experiment is suggested for direct test of the driven‐wave hypothesis discussed herein.