In this article, we report the fabrication of an oxygen sensor based on gallium oxide thin films prepared by the chemical solution deposition method. The oxygen sensing properties of the device (response and recovery time, sensitivity, stability) were investigated at high temperature (1000 °C). Attention was paid to the influence of the annealing conditions on the oxygen sensing properties of the Ga2O3 thin film sensor. Surface morphology of thin film samples, studied by using atomic force microscopy measurements, shows that there are grain size differences due to various annealing conditions (temperature and time period). The results show that the annealing conditions affect not only the grain size and surface structure of Ga2O3 thin films but also the oxygen sensing properties of the material. It was shown that the response time of the Ga2O3 sensors decreased when the annealing temperature and annealing time were increased. The sensitivity of the sensor devices it is also affected when the annealing parameters are changed. The response time at 1000 °C was 12 s for the Ga2O3 film sensor annealed at 1000 °C for 14 h.