A push-push transformer-based voltage-controlled oscillator (VCO) is proposed and analyzed to achieve high efficiency and a wide tuning range at sub-terahertz (THz) frequencies. Analyses show that the coupling factor of the transformer to obtain high output power (P-OUT) has to be carefully chosen by consideration of gate-to-drain voltage gain as well as matching impedances seen from the drain and to the gate of a VCO transistor at the 2nd harmonic frequency (2f(o)). Analysis also shows that the transformer-based resonator allows a wide tuning range. In addition, it has been shown that the introduction of a parallel inductor to a varactor leads to high P-OUT and low phase noise. The proposed 239 GHz VCO with a 65 nm CMOS process demonstrates the high efficiency of 1.45%, the P-OUT of -4.8 dBm, and the wide tuning range of 12.5% with a supply voltage of 1.2 V.