Single-atom catalysis has emerged as a novel approach to combine the single-site nature of homogeneous catalysis with the robustness and technical readiness of heterogeneous catalysis. The so-called single-atom catalysts are mono-dispersed metal atoms/ions stabilized by a specific support, often via coordination to carbon and heteroatoms such as O, N, P, S, etc. These catalysts are attractive because they exhibit maximum atomic efficiency and possess welldefined active sites, which facilitate mechanistic understanding. Although a number of singleatom catalysts have been developed for electrochemical reactions, only few single-atom OER catalysts are reported. OER in alkaline medium is an archetypical electrochemical reaction for which bimetallic catalysts are often more active than monometallic catalysts. Based on time-resolved spectroscopic measurements of intermediates in a photochemical OER catalyzed by Co3O4, Frei and co-workers proposed that an oxygen-bridged Co-Co moiety reacted much faster than a single Co site. Moreover, benchmark works showed that the nickel-iron oxide (NiFeOx) and cobalt-iron oxide (CoFeOx) are among the most active OER catalysts in alkaline solutions. Incorporation of a trace amount of iron will significantly enhance the activity of pure NiOx and CoOx, while pure FeOx has low activity. It follows that instead of single-atom catalysts, a so-called double-atom catalyst would be more active for OER. Recently, some double-atom catalysts were reported to exhibit higher activity than single-atom catalysts for electrocatalytic oxygen reduction reaction (ORR), CO2 reduction, and cocatalyst for photoanode. However, double atom OER electrocatalysts composed of only earth-abundant elements are hitherto unknown. Herein, we report such a Co-Fe double-atom catalyst, which exhibits turnover frequencies (TOFs) comparable to state-of-the-art OER catalysts. This double-atom catalyst is generated from a single-atom Co pre-catalyst (Co-N-C) via an in-situ electrochemical method. The work presents, to our knowledge, the first example of double-atom electrocatalyst that is highly efficient for OER.