Hydrogen generation via photoelectrochemical (PEC) overall water splitting is an attractive means of renewable energy production so developing and designing the cost-effective and high-activity bifunctional PEC catalysts both for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) has been focused on. Based on first-principles calculations, we propose a feasible strategy to enhance either HER or OER performance in the monoclinic exposed BiVO₄ (110) facet by the introduction of oxygen vacancies (O_vacs). Our results show that oxygen vacancies induce charge rearrangements, which enhances charge transfer between active sites and adatoms. Furthermore, the incorporation of oxygen vacancies reduces the work function of the system, which makes charge transfer from the inner to the surface more easily; thus, the charges possess stronger redox capacity. As a result, the O_vac reduces both the hydrogen adsorption-free energy (ΔG_H*) for the HER and the overpotential for the OER, facilitating the PEC activity of overall water splitting. The findings provide not only a method to develop bifunctional PEC catalysts based on BiVO₄ but also insight into the mechanism of enhanced catalytic performance.