The oxygen evolution reaction (OER, 2H2O → O2 + 4H+ + 4e–) plays an essential role in energy conversion electrochemical processes from hydrogen production via water splitting to electrosynthesis of high-valued chemicals in aqueous solutions. Unfortunately, the OER is notorious for its sluggish kinetics. A key source of its kinetic barrier is that the OER is a multi-electron transfer reaction. Therefore, an effective OER electrocatalyst needs to stabilize multiple intermediates via surface adsorption (e.g. OHad, Oad, OOHad). This work focuses on the fundamental understandings of these electroadsorption steps and their impacts on the OER electrocatalysis to develop the principles for the OER catalyst optimization. Our approach extracts the electroadsorption energy on the thin-film transition-metal oxides grown epitaxially using molecular-beam epitaxy (MBE). The well-defined surfaces enable us to benchmark our measurements against the computational model systems. We demonstrate that the OHad and Oad energies are linearly related, which provides the first known experimental evidence behind the concept of the scaling relation in catalysis. Building on this scaling relation, we use an oxygen adsorption energy (ΔGO-ΔGOH) to approximate the activation energy of the OER and examine its correlation to the OER kinetics by modifying the material electronic structure and interfacial environment. In addition, we study the mechanism of electron and proton transfer in electroadsorption steps by measuring their kinetics in different electrolytes. These fundamental insights provide the guidelines to optimizing the material properties and interfacial environment for the OER electrocatalysis.