The oxygen evolution reaction (OER) from water is a critical component of a sustainable energy future; however, its mechanism has proved difficult to identify experimentally. This complexity is due to the elusive nature of electron and proton transfer intermediates that form within an interfacial water network and are buried at the solid–liquid interface. Here, we summarize recent measurements identifying the first two electron and proton transfer intermediates, e.g., OH∗ and O∗, on metallic oxide surfaces prior to the OER cycle, and the first electron and proton transfer intermediate, OH∗, as a metastable species during OER on a photo-excited, semiconducting oxide surface.