Achieving an efficient and stable oxygen evolution reaction (OER) in an acidic or neutral medium is of paramount importance for hydrogen production via proton exchange membrane water electrolysis (PEM-WE). Supported iridium-based nanoparticles (NPs) are the state-of-the-art OER catalysts for PEM-WE, but the nonhomogeneous dispersion of these NPs on the support together with their nonuniform sizes usually leads to catalyst migration and agglomeration under strongly corrosive and oxidative OER conditions, eventually causing the loss of active surface area and/or catalytic species and thereby the degradation of OER performance. Here, we design a catalyst comprising surface atomic-step enriched ruthenium-iridium (RuIr) nanocrystals homogeneously dispersed on a metal organic framework (MOF) derived carbon support (RuIr@CoNC), which shows outstanding catalytic performance for OER with high mass activities of 2041, 970 and 205 A g(RuIr)(-1) at an overpotential of 300 mV and can sustain continuous OER electrolysis up to 40, 45, and 90 h at 10 mA cm(-2) with minimal degradation in 0.5 M H2SO4 (pH = 0.3), 0.05 M H2SO4 (pH = 1), and PBS (pH = 7.2) electrolytes, respectively. Comprehensive experimental studies and density functional theory (DFT) calculations reveal that the good performance of RuIr@CoNC can be attributed, on one hand, to the presence of abundant atomic steps that maximize the exposure of catalytically active sites and lower the limiting potential of the rate-determining step of OER and, on the other hand, to the strong interaction between RuIr nanocrystals and the CoNC support that endows homogeneous dispersion and firm immobilization of RuIr catalysts on CoNC. The RuIr@CoNC catalysts also show outstanding performance in a single-cell PEM electrolyzer, and their large-quantity synthesis is demonstrated.