Water electrolysis driven by electrical power generated from renewable energy sources will play a pivotal role in future sustainable societies, which requires adaptation of various reaction conditions as well as electrolyte identities. Regardless, the anodic half-reaction of the oxygen evolution reaction (OER) is considered a kinetic bottleneck. This study provides quantitative description of the OER kinetics based on rigorous microkinetic analyses including Tafel analysis, isotope effects and temperature dependence using an IrOx electrocatalyst in unbuffered solution at varying pH levels. The diffusional constraints of H+/OH− determine three distinctive kinetic regimes in the pH-potential-current relationships: below pH 5, between pH 5 and 10, and above pH 10 at appreciable current densities on the order of 1 mA cm−2. When shifting from alkaline to acidic solution, the complete consumption of local OH− near the electrode surface switches the OER proceeding as the oxidation of OH− to that of the water molecule at pH ∼11 irrespective of the electrode identity. At pH 5-10, the diffusional constraints of H+ generated via oxidation reaction yield an environment with pH ∼4 near the electrode surface even prior to the OER, resulting in a bulk pH-independent region for the OER performance. Under this unbuffered near-neutral-pH condition, the isotope effect was diminished for the OER catalysis, which is consistent with the rate-determining step (rds) being the sole electron-transfer step via the formation of O-O bonds, decoupled from proton transfer. This reaction mechanism is distinct from that under more acidic conditions (pH < 4), although the water molecule is the same reactant. Under acidic conditions, noticeable isotope effects were observable, which is consistent with the formation of O-O bonds being the rds on uncoordinated bare Ir sites as the most abundant surface species. This study provides a quantitative description of the reactant- and mechanistic-switching that points to concurrent optimization of both electrode materials and electrolyte for improved OER performance at near-neutral pH levels.