Water oxidation is an important first step in photosynthesis. It provides electrons and protons for reduction reactions such as hydrogen generation or carbon dioxide reduction. From a technological development perspective, it is critically important to carry out water oxidation on heterogeneous catalyst surfaces. However, this reaction is exceedingly difficult to study. Part of the reason for the challenge lies in the lack of clarity of the catalytically active centers for water oxidation. Another reason for the difficulty comes from the fact that the structures of the active center often evolve under reaction conditions. These issues have made water oxidation by heterogeneous catalysts poorly understood. In this talk, we present our recent efforts in addressing this challenge. Using cobalt oxide-based catalysts as a model system, we introduced a new approach of varying water activities. This was achieved through a "water-in-salt" electrolyte system. It was observed that such an electrolyte allows for discerning two competing mechanisms of water oxidation, namely water nucleophilic attack and intramolecular oxygen coupling. Moreover, it was found that the mechanisms are sensitive to the driving forces, demonstrating a preferences to water nucleophilic attack at high applied potentials and intramolecular oxygen coupling at low applied potentials. The results have major implications to the catalyst design for water oxidation reactions.