Transition-metal oxide surfaces play an important role in a wide range of applications, e.g. heterogeneous catalysis, photoelectrolysis, biocompatibility and sanitary disinfection. Defects like oxygen vacancies often dominate electronic and chemical properties of transition-metal oxide surfaces. In recent studies on a prototypical model oxide system (rutile TiO2(110) surface) we exploited our high-resolution, variable-temperature and fast-scanning Aarhus STM to study how oxygen vacancies influence surface and interface reactions. Water dissociation on TiO2 is of fundamental interest as an example of a simple surface chemical process with important applications. In high-resolution STM experiments, we unambiguously identify surface oxygen vacancies and hydroxyl groups. Controlled voltage STM pulses allow us to desorb OH groups only; O vacancies remain unaffected. Through STM images and time-resolved movies, we determine the active site responsible for the water molecule dissociation on TiO2(110). At low H2O exposures, O vacancies dissociate water molecules by transferring one proton to a nearby oxygen atom, forming two hydroxyl groups for every vacancy. At elevated exposures, a novel water dissociation channel is seen and will be described in detail. The amount of water dissociation is not limited by the density of oxygen vacancies on the clean surface as proposed earlier in literature. Extended oxygen exposure on TiO2(110) will lead to restoichiometrization of the support, thus markedly reducing its reactivity. An atomic-scale understanding of the healing process is still lacking, but it is thought to be a simple mechanism where a single O2 molecule heals two vacancies subsequent to a dissociative process. Using high-resolution STM and TPD measurements, we investigate the interaction of O2 from the gas phase with different surface defects (O vacancies, OH groups) on TiO2(110).