A push-push transformer-based oscillator (TBO) adopting a power leakage suppression technique has been proposed. The proposed technique reduces the power loss due to unwanted leakage path without additional DC power consumption, hence improving the output power and DC-to-RF efficiency. The measured output power of the proposed single core oscillator is -4.5 dBm at 270 GHz with 2.1% DC-to-RF efficiency.