We present a low-power low-frequency oscillator that is fully integrated on a bendable plastic substrate using amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs). Its purpose is the duty cycling of components of a wireless sensor tag to realize power savings. In addition, the oscillator can directly be used as a light sensor. It oscillates between 5 Hz in the dark and 31 Hz under daylight, from a 5-V supply voltage. The measured light-sensitivity of the oscillation frequency is between 7.4 Hz/klx in the dark and around 1.7 Hz/klx in daylight. On average, the frequency of oscillation changes by 58 %/klx. The required power is 188 μW. The presented design is a combination of the inductance-capacitance cross-coupled oscillator structure and two single-transistor active inductors, which enable high gain at low power levels in a small chip area. We analyze the circuit and derive design guidelines for minimizing the oscillation frequency, circuit area, and power consumption. Finally, we report the measurements including jitter and deduce implications for the accuracy of light measurements.