Oxygen vacancies play an important role in the performance improvement of oxide semiconductors as photoanodes for water splitting, such as TiO2, WO3, and Fe2O3. Conductivity improvement due to the presence of oxygen vacancies was reported to be the main reason for the enhanced performance. However, oxygen vacancies may also affect light absorption and charge transfer through the solid/electrolyte interface. The roles of oxygen vacancies have not been thoroughly discussed in the past. Herein, with hematite as an example, the effects of oxygen vacancies on bulk charge transport and surface catalysis are quantitatively analyzed by decoupling photon absorption, interfacial charge transfer and charge separation processes. Oxygen vacancies improve the charge separation of both pristine and Ti-doped hematite. However, opposite observations are found in the charge transfer process for pristine and Ti-doped hematite: the positive effect in pristine hematite but the negative effect in the Ti-doped one. An electrochemical technique is used to analyze the different influences on pristine and Ti-doped hematite to unravel the mechanism of the opposite observations caused by oxygen vacancies. The current study sheds lights on how oxygen vacancies affect various aspects of important factors behind PEC performance, which is helpful to the development of more efficient photoanodes in the future.