A frequency-locked loop (FLL) is proposed to provide a real-time frequency calibration for OOK power oscillator transmitters. This FLL incorporates a pulse-width detector (PWD) based on Time-Registers (TRs) to discriminate accurately between the carrier frequency and the preset target frequency during the data modulation. As a result, frequency calibration can be performed simultaneously with data modulation without interruption. A prototype of an OOK power oscillator transmitter integrated with the proposed FLL is implemented with onboard discrete electronic elements for demonstration. The measurement indicates that the FLL can accurately lock to a carrier frequency of 160MHz despite electromagnetic (EM) interference in the loop antenna of the transmitter. In contrast, with EM interference and charge leakage, the carrier frequency differs from the expected value by about 1 MHz when the FLL is disabled.