In this study, Ga2O3 films were deposited on sapphire substrate using Radio Frequency (RF) magnetron sputtering technique. The films were produced at 100 W power and at different oxygen percentages of 0%, 2% and 4%. Then, the films were annealed in air at 900°C. The transmittance measurements of all films were performed and the energy band gaps were calculated. The energy band gap between before and after annealing increased as the oxygen percentage increased. Based on these results, it was revealed that the O2 concentration plays a crucial role in controlling the optical properties of Ga2O3, which can greatly affect the device performance.