A general model of harmonic oscillator is presented. Based on this model, several sources of power loss in harmonic oscillator can be identified. To optimize the output power, fundamental design principle of harmonic oscillator is also presented. Applying this design principle can help to eliminate power losses and effectively increase the output power and DC-to-RF efficiency of harmonic oscillator. A push-push transformer-based oscillator (TBO) topology is adopted as high-power sub-terahertz source in this work. Theoretically, this topology can generate the highest possible harmonic power from a certain transistor. Using 65-nm CMOS technology, a 261 GHz push-push transformer-based oscillator is fabricated. Measurement result shows an output power of -4.7 dBm can be achieved by this oscillator while drawing 15 mA from 1.2 V supply. As the result, this work demonstrates the highest DC-to-RF efficiency among state-of-the-art oscillator around 250 GHz.