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This study investigated the effects of oxygen (O₂) flow rates and annealing temperatures on optical, electrical, and structural properties of indium–gallium–zinc oxide (IGZO) film on glass substrates fabricated by using a co-sputtering system with two radio-frequency (RF) (In₂O₃ and Ga₂O₃) and one direct current (DC) (Zn) magnetron. The average transmittance and optical energy gap increased significantly when the oxygen flow rate was increased from 1 sccm to 3 sccm. An increased O₂ flow during co-sputtering IGZO films caused the crystallinity of the InGaZn₇O₁₀ phase to increase, yielding a smoother and more uniform granular structure. The carrier mobility rose and the carrier concentration decreased with increasing O₂ flow. The results of X-ray photoelectron spectra (XPS) analyses explained the impacts of the O₂ flow rates and annealing temperatures on optical and electrical properties of the co-sputtered IGZO films. The optimum process conditions of the co-sputtered In₂O₃-Ga₂O₃-Zn films were revealed as an O₂ flow rate of 3 sccm and an annealing temperature at 300 °C, which showed the largest average transmittance of 82.48%, a larger optical bandgap of 3.21 eV, and a larger carrier mobility of 7.01 cm² V⁻¹s⁻¹. XPS results at various annealing temperatures indicated that the co-sputtered IGZO films with an O₂ flow rate of 3 sccm have more stable chemical compositions among different annealing temperatures.