This paper proposes a Class E/F$_3$ tuned power oscillator based on the Class E/F$_3$ amplifier. The oscillator has a feedback-loop circuit and starts the oscillation in automatic. In addition, the oscillator achieves zero-voltage switching and zero-voltage derivative switching; therefore, the switching losses are reduced under high oscillation frequency. Moreover, the third harmonic tuned by providing a third harmonic series resonant circuit reduces the switch voltage stress of the <sc>mosfet</sc> sufficiently. The oscillator was designed, built and tested using an IRFR120Z power <sc>mosfet</sc>. Under the condition of the input voltage 6.003 V, following results were shown: the measured oscillation frequency was 801.6 kHz, the output power was 0.9075 W, and the power conversion efficiency was 86.24%.