In order to improve poor charge carrier transport properties of p-type oxide semiconductors, the post thermal treatment under oxygen atmosphere is systematically studied as a defect engineering strategy for LaFeO₃ photocathodes. The post-thermal oxidation process significantly decreases the photo-generated charge recombination and promotes the charge transport properties of LaFeO₃ photocathodes. The density functional theory calculations and experimental analyses indicate that the thermal oxygen treatment can promote the formation of La vacancy and the reduction of the oxygen vacancy in LaFeO₃, which facilitates its bulk charge transport by increasing hole carrier density. The optimized LaFeO₃ photocathode yields an O₂ reduction photocurrent density of −313 µA/cm² at 0.6 V vs RHE in O₂-saturated KOH solution under 1 sun irradiation. Additionally, we also investigated the feasibility of photoreduction of water by using a Pt-deposited LaFeO₃ photocathodes.