This article presents a high-voltage switched oscillator (SO) with a quarter-wave open-circuited stub (OCS) for generating mesoband high-power microwave (HPM) pulses. Unlike the conventional SO, the proposed SO generates mesoband HPM pulses whose dc offset is greatly reduced because the OCS inserted between a charged transmission line and an antenna blocks the flow of the electric energy from a high-voltage power supply to the antenna. Thus, the proposed SO allows the insulation capability of the antenna to be increased and ground-loop-integrated-wideband antennas to be employed. The transient response of the proposed SO is theoretically analyzed to understand its operation mechanism. The results prove that the proposed SO is capable of producing mesoband HPM pulses with a damped rectangular pulse waveform without a high-voltage dc offset. Also, the amplitude and quality factor are discussed. Compared with the conventional SO, the proposed SO has a larger output amplitude and smaller quality factor. Here, the difference between the quality factors is negligible. In addition to the theoretical analysis, we simulated and fabricated the proposed SO. Consequently, the results of the circuit simulation and experiment demonstrated the validity of the proposed design concept.