Photostability is one of the most essential properties for evaluating photoelectrochemical (PEC) water splitting performance on semiconductors. Herein, the oxygen‐deficiency conditions are applied to tune and activate BiVO4 photoanodes with a class of oxygen vacancies across the whole bulk material, and regulate the electronic occupancy of these states upon the charge carrier processes that determine PEC water oxidation activity. Through the experimental results and nonadiabatic molecular dynamics with time‐domain density functional theory calculations, the charge carrier lifetime can be influenced by the oxygen vacancies concentration on BiVO4, and the semiconductor can be flexibly photoactivated under oxygen‐sufficient and deficient atmospheres for enhancing the charge carrier density and photovoltage. The PEC performance of BiVO4 is further boosted by Pt doping, which exhibits a record photocurrent density of 5.45 mA cm–2 at 1.23 VRHE with solar conversion efficiency of 2.1% at 0.65 VRHE. The Pt can prevent the unnecessory charge recombination on the defected BiVO4, which also enhances the majority charge carrier density, resulting in one of the best charge separation efficiencies, close to 100%, among the steady‐state PEC performance for BiVO4. More importantly, the resulting Pt:BiVO4 presents long‐term stability over 50 h at 0.8 VRHE. Herein, oxygen‐deficiency conditions are applied to tune and activate BiVO4 photoanodes with a class of oxygen vacancies across the whole bulk material, and regulate the electronic occupancy of these states upon the charge carrier processes that determine photoelectrochemical (PEC) water oxidation activity. The material is doped with Pt, which further enhances the majority charge carrier density, resulting in one of the best charge separation efficiencies, close to 100%, among the steady‐state PEC performance for BiVO4 photoanodes.