Experimental study on high-power subterahertz waves generated by an overmoded surface wave oscillator (SWO) is carried out in this paper. With accurate fabrication and assembly, the frequency obtained is 0.148 THz as measured by the cut-off waveguide and subharmonic heterodyne methods, and the output power is about 5 MW with the far-field measurement method at a diode voltage of 300 kV, beam current of 2.3 kA, and guiding magnetic field of 5 T. The frequency of the device remains fairly stable when the voltage of the diode changes from 200 to 330 kV. This is a typical feature of the SWO, and is verified by this experiment. The start-up time of the device is as small as 1 ns, which is suitable for use of the device as a short-pulse accelerator with a compact structure.