This paper presents an On-Off-Keying (OOK) modulator for a flexible and wearable wireless transmitter implemented in an amorphous-Indium-Gallium-Zinc-Oxide (a-IGZO) TFT technology. The circuit consists of a three-stage ring oscillator for the carrier and an output driver with an OOK modulation switch, realized with just five transistors. In order to maximize the operation frequency, we use 2 μm-long nMOS transistors in the circuit design. The proposed OOK modulator is fabricated on a polyimide flexible substrate, and characterized with 3-to-5 V supply voltages and an output load capacitance of 15 pF. The circuit operates from the lowest supply voltage of 3 V, while the highest measured oscillation frequency is 3.76 MHz at 5 V VDD. Although the schematic is simple and straight forward, the equivalent modulation depth ranges from 61.3 % to 78.2 %, which can be detected with an existing AM/OOK receiver in the same technology. The power consumptions for 3 V and 5 V supply voltages are 2.15 mW and 6.77 mW, respectively.