In this work, BiOCl samples were prepared by a hydrothermal method and OVs were introduced by tuning annealing temperature and gas atmosphere. The formation process of OVs is investigated by in-situ diffuse reflectance FT-IR spectroscopy (DRIFTS) experiments. X-ray photoelectron spectra (XPS) show the absence of surface OVs in as-prepared BiOCl samples, while electron paramagnetic resonance (EPR) confirms the presence of (bulk) OVs. The influence of oxygen vacancies in light absorption, carrier concentration and band alignment are further investigated and discussed.