Ligand effects of some representative monomeric Ru-based water oxidation catalysts on the key O-O formation step are revealed in this work. Three effects, namely, cis-effect, net charge effect, and steric hindrance effect, are identified, which can exert sizable modulation on the O-O formation barriers for the two widely accepted O-O formation mechanisms of WNA (water nucleophilic attack) and I2M (direct coupling of two high-valent metal oxo units). The study demonstrates that, through the way of ligand design, there remains a large space for improving O-O bond formation reactivity.