  BiVO$_4$ is a promising photocatalyst for efficient water oxidation, with
surface reactivity determined by the structure of active catalytic sites.
Surface oxidation in the presence of oxygen vacancies induces electron
localization, suggesting an atomistic route to improve the charge transfer
efficiency within the catalytic cycle. In this work, we study the effect of
oxygen vacancies on the electronic and optical properties at BiVO$_4$ surfaces
upon water oxidation. We use density functional theory and many-body
perturbation theory to explore the change in the electronic and quasiparticle
energy levels and to evaluate the electron-hole coupling as a function of the
underlying structure. We show that while the presence of defects alters the
atomic structure and largely modifies the wavefunction nature, leading to
defect-localized states at the quasipatricle gap region, the optical
excitations remain largely unchanged due to substantial hybridization of defect
and non-defect electron-hole transitions. Our findings suggest that
defect-induced surface oxidation supports improved electron transport, both
through bound and tunable electronic states and via a mixed nature of the
optical transitions, expected to reduce electron-hole defect trapping.
