Some key issues associated with the interaction efficiency of a harmonic gyrotron oscillator operating in the millimeter wave regions were studied. Operating at the third-harmonic, the required magnetic field was reduced to 1.185 T for a 94GHz gyrotron oscillator, which makes it possible to replace the superconducting magnet by a permanent magnet. A self-consistent code and a particle-in-cell(PIC) software were used to investigate the harmonic beam-wave interaction. The interplay between the cavity quality factor and interaction efficiency was revealed, and the dependence of both beam voltage and electron beam pitch factor on the coupling coefficient was also studied. Through carefully choosing operating mode, optimizing system parameters, and using linearly increased axial magnetic field, the output power of 95kW and efficiency of 19.7% was achieved under accelerating voltage of 40kV, beam current of 12 A, and transverse velocity spread of 3%. The efficiency can be further increased to 39. 2% by utilizing a single stage depressed collector(SDC).