The rational design of efficient electrocatalysts for the oxygen evolution reaction (OER), holds the key to advancing the overall electrolytic water splitting performance. Here, a scalable one-pot synthesis of a stable chromium─iron nickel boride (Cr─FeNiB) electrocatalyst is reported for OER in which nickel-boron sites are micro-environmentally modified through interactions with iron and chromium. Comprehensive, correlative electrochemical, structural, and chemical analyses reveal the formation of amorphous-crystalline core-shell structures that transform into nanosheets upon activation with enhanced water oxidation catalytic activity. The enhanced catalytic performance is attributed to the chromium-induced chemical self-reconstruction of the catalyst, which facilitates favorable OER kinetics, increased turnover frequency, and a synergistic effect between metal and boron constituents. Density Functional Theory (DFT) calculation showed that chromium incorporation effectively shifts the d-band centers (ɛd) closer to the Fermi level and narrows the metal-d/boron-p band center gap (Δɛd-p) (Ni2B 1.10 eV → FeNiB 0.71 eV → Cr─FeNiB 0.55 eV) ultimately enhancing OER activity. Accordingly, the Δɛd-p is established as a key electronic descriptor for predicting and optimizing OER performance. These findings pave the way for a better understanding of metal boride-derived electrocatalysts and also contribute to the development of efficient, stable, earth-abundant, non-noble metal catalysts for water oxidation.