Exploring high-performance and cost-effective electrocatalysts that are applicable in oxygen evolution reaction (OER) is crucial for water splitting and energy storage. In this work, a facile and scalable chemical reduction strategy is developed to synthesize FeCoNiPB non-noble metal-based amorphous high-entropy oxides for the OER in alkaline media. The FeCoNiPB oxides exhibit overpotentials of 235 and 306 mV at current densities of 10 and 100 mA/cm2, respectively, as well as a small Tafel slope of 53 mV/dec in 1.0 M KOH solution, outperforming the performance of FeCoPB, FeNiPB, and CoNiPB oxides and the commercial RuO2, while maintaining excellent stability with negligible overpotential amplification over 40 h. The superior OER electrocatalytic efficiency and stability of the FeCoNiPB catalyst is primarily attributed to its unique amorphous high-entropy nanostructure, synergistic effect of the multiple components, and in situ-formed amorphous sheets with a thin (FeCoNi)OOH crystalline layer on the edge during long-term OER. This work provides new insights to design and prepare low-cost, highly efficient, and durable OER electrocatalysts.