Undoped ZnO thin films were electrochemically fabricated on InSe:Ho substrates. Their structural, electrical properties were investigated by X-ray diffraction (XRD), Atomic Force Microscopy (AFM) and current-voltage (I-V) measurements. The responses to oxygen were scrutinized by resistance measurement at various temperatures ranging from 50 °C to 275 °C exhibiting that the undoped electrochemically deposited ZnO can reliably be used as O₂ gas sensor. It is evidently seen that samples respond differently to O₂ in terms of recovery and response time while their gas responses have been found to be linearly increased with operating temperature. The gas responses have also one to one correlation with the grain size of the ZnO thin films. The best response time and recovery time have been found to be 79 s and 35 s for the ZnO thin films with the Lotgering factor of 0.85 and thickness of 189 nm.