Oxygen vacancies in the a-Ga₂O₃ film play a crucial role in the performance of a-Ga₂O₃-based solar-blind photodetectors (SBPDs). The concentration of oxygen vacancies is a “double-edged sword”, increasing the responsivity of the SBPDs but deteriorating the response speed. In order to achieve a balance between these two key parameters, we adjusted the oxygen vacancy concentration in the film by delicately tuning the oxygen partial pressure during the sputtering process. The metal-semiconductor-metal (MSM) SBPDs were prepared accordingly. The results demonstrate that incorporating moderate oxygen can reduce the oxygen vacancies in the film and improve the density of the film. Under appropriate conditions, the oxygen partial pressure enables the photodetector to maintain good responsivity while having a fast response speed. At an oxygen partial pressure of 3%, the device has a high responsivity of 2.62 A/W under 254-nm DUV irradiation and a fast response speed of 2.2 s/0.96 s.