Indium zinc oxide (IZO)-gated AlGaN/GaN high electron mobility transistors (HEMTs) were used to detect oxygen gas. Amorphous IZO films with high carrier concentration of 1021 cm-3 were deposited on the gate region of the HEMTs by cosputtering from ZnO and In2O3 targets. The changes in IZO gated-AlGaN/GaN HEMT drain current were used to monitor the presence of oxygen. The IZO gated AlGaN/GaN HEMT sensors were tested with O2 at room temperature, 50 °C, and 120 °C. There was no response to O2 at room temperature. At 50 °C, the sensors could sense O2 but gradually saturated. The sensor showed a strong response to the oxygen gas at 120 °C, which is a much lower temperature than with conventional oxide-based oxygen sensors that typically operate in the range of 400–700 °C. This enhanced oxygen sensing sensitivity was due to the amplification effect of the AlGaN/GaN HEMT. A preannealing step at 350 °C was also found to improve the sensitivity and response time of O2 sensing at 120 °C.