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.