This brief reports a differential complementary drain-grounded voltage-controlled oscillator power amplifier (DG-VCO-PA) achieving 7.2% power efficiency (ηTX) when transmitting 400 kbps on-off-keying (OOK) symbols at –12 dBm equivalent isotropic radiated power (EIRP). Benefiting from the drain-grounded operation, the negative transconductance cell of the DG-VCO-PA can stay in the saturation region until the output amplitude reaches rail-to-rail. Therefore, the DG-VCO-PA exhibits 2.8× higher ηTX and 6 dB higher output power than a cross-coupled VCO-PA when delivering a rail-to-rail output. Fabricated in 65 nm CMOS, the DG-VCO-PA can radiate –16.6 dBm to –12 dBm EIRP at 900 MHz without reliability issues by driving a 15 mm square loop antenna. The high-Q antenna and switched capacitors grant the DG-VCO-PA a low phase noise of –125.8 dBc/Hz at 1 MHz offset frequency and only 10 kHz frequency drift within 10 seconds.