This paper proposes a frequency-domain OOK (F-OOK) modulation transmitter with a phase rotator and a 4-stage ring digital/voltage-controlled oscillator (D/VCO) to achieve high energy, bandwidth and area efficiencies. Both frequency deviation and frequency modulation are realized in the phase domain by utilizing the phase rotator. A 9-bit phase rotator covering a phase shifting range of 180° is designed with a phase resolution of 5.625° and phase steps of 32. A hybrid 2.4GHz PLL with a 4-stage ring D/VCO generates orthogonal signals as inputs of the phase rotator. The proposed F-OOK transmitter designed in 65nm CMOS consumes 5.4mW from a 1V supply at the maximum data rate of 10Mb/s, achieving an energy efficiency of 0.54nJ/bit.