The oxygen evolution reaction (OER) is an essential anode reaction for the generation of fuels through water splitting or CO2 electroreduction. Mixed metal oxides containing Co, Fe or Ni have proved to be the most promising OER electrocatalysts in alkaline media. However, the active sites and reaction mechanisms of these catalysts are difficult to study due to their heterogeneous nature. Here we describe a general synthesis of Co-, Fe- and Ni-containing double-atom catalysts from their single-atom precursors via in situ electrochemical transformation. Characterization reveals molecule-like bimetallic active sites for these supported catalysts. For each catalyst, we propose a catalytic cycle; all exhibit bimetallic cooperation and follow a similar O-O bond-forming step. However, the mechanisms diverge in the site and source of OH- for O-O bond formation, as well as the order of proton and electron transfer. Our work demonstrates double-atom catalysts as an attractive platform for fundamental studies of heterogeneous OER electrocatalysts., Oxygen evolution reaction (OER) catalysts often comprise multiple metal ions in various configurations, hampering mechanistic understanding of how catalysis proceeds. Now, researchers prepare a series of double-atom OER catalysts based on Ni, Fe and Co, which act as molecular-like models and are more amenable to mechanistic study.