Competitive oxygen kinetic isotope effects (¹⁸O KIEs) on water oxidation initiated by ruthenium oxo (RuO) complexes are examined here as a means to formulate mechanisms of O–O bond formation, which is a critical step in the production of “solar hydrogen”. The kinetics of three structurally related catalysts are investigated to complement the measurement and computation of ¹⁸O KIEs, derived from the analysis of O₂ relative to natural abundance H₂O under single and multi-turnover conditions. The findings reported here support and extend mechanistic proposals from ¹⁸O tracer studies conducted exclusively under non-catalytic conditions. It is shown how density functional theory calculations, when performed in tandem with experiments, can constrain mechanisms of catalytic water oxidation and help discriminate between them.