The power capacity of high-power microwave generating devices is improved by increasing the overmode ratio of the slow wave structure. The nested structure allows the use of the hollow or inner conductor structure of the overmode device, while the low impedance of the nested structure device makes it a good match for low-impedance pulsed power sources. A high-power microwave oscillator with locked frequency and phase is proposed based on the nested structure. Compared with the traditional method to lock phase and frequency, a method based on coupling waveguide is proposed to realize the locked frequency and phase. The microwave generated by the outer relativistic klystron oscillator (RKO) or inner RKO leaks into the other RKO through the coupling waveguide to premodulate the electron beam to lock phase and frequency. In addition, a dual-way power combiner is designed at the operating frequency of the oscillator to realize the combination of the inner and outer high-power microwave. The power combiner can make up for the phase difference between the two output ways, increasing the power combination efficiency to 98.3%. When the diode voltage is 575 kV and the magnetic field is 0.6 T, the output power of inner and outer RKOs are 2.2 GW and 3.2 GW, respectively, with the frequency difference fluctuating less than 20 MHz and the phase difference stabilized near 10°; loading the designed power combiner, high power microwave with power of 5.31 GW and efficiency of 32.2% is obtained. The results show that the oscillator saturation time is shortened and the output power is increased when the nested device is in the locked state.