Oxygen evolution reaction (OER) via mixed metal oxy hydroxides [M(O)(OH)] may take place on a large variety of possible active sites on the actual catalyst. A single site computational description assumes a 4-step electrochemical mechanism with coupled H+/e− transfers between 4 intermediates (M-*, M-OH, M=O, M-OOH). We also consider bifunctional routes, in which an unstable M-OOH species converts via a proton shuttling pathway to a thermodynamically more favourable bare M-* site, O2 and a hydrogenated acceptor site; the acceptor site takes up the proton forming a hydrogenated acceptor site after recombination with an electron from the catalyst material. Here, we combine pure metal γ-M(O)(OH) edge sites (M=Fe, Co, Ni) with as proton-acceptor sites different threefold coordinated oxygens on β-(M,M’)(O)(OH) terraces (M,M’=Fe, Co, Ni). The acceptor sites on these terraces have of a M’2MO motif. Our combinatorial study results in a ranking of the bifunctional OER activity on a 3D-volcano plot. By studying various bi- and tri-metallic oxy hydroxide combinations, we show that their excellent experimental OER activity results from bifunctionality and provide a roadmap to construct innovative low overpotential OER catalysts.