A 40-GHz hybrid class-AB/class-B CMOS VCO with a current-combining transformer (CCT) using a 65-nm RF CMOS process is presented. The hybrid class-AB/class-B configuration is adapted to two cross-coupled pairs to lower the power consumption while maintaining the phase noise performance. As a simple coupling network between two cross-coupled pairs with different bias conditions, the CCT performs the roles of a high Q-factor resonator, a large impedance transformer, and a buffer without additional circuitry. The VCO operates at 38.15-40 GHz with power consumption in the range of 11-13 mW. The measured phase noise at a 1-MHz offset from the 40 GHz carrier is -117.1 dBc/Hz with a 1.0 V supply voltage. The proposed power oscillator achieves an FOMT value of -191.3 dBc/Hz.