A high-efficiency high-power millimeter-wave oscillator was implemented in IBM 45 nm silicon-on-insulator (SOI) CMOS using a class-E power amplifier in a feedback configuration. The oscillator achieves a peak output power of +4.6 dBm, a peak efficiency of 11.54%, and phase noises of -106.61 dBc/Hz and -131.28 dBc/Hz at 1 and 10 MHz offsets, respectively. All matching circuits and inductive elements are realized on-chip with transmission line for application in the millimeter-wave frequency bands. The oscillator is tunable between 41.08 GHz and 42.87 GHz by using a λ/4 stub terminated with a varactor in the feedback path. The circuit achieves the highest reported efficiency and output power for silicon-based monolithic millimeter-wave oscillators to the best of the authors' knowledge.