Electrochemical water splitting in acidic media is a promising approach for generating hydrogen (H₂) gas as a green fuel. In this regard, catalyst design for the oxygen evolution reaction (OER) is more demanding due to the slow transfer kinetics of involved four electrons and high corrosion and degradation of the electrode. Besides, a sustainable OER catalyst should be noble metal-free and have long-term durability under positive potentials in acidic solutions. The current study suggests a nickel-manganese-antimony electrocatalyst synthesized via a facile thermal decomposition method for OER in 0.5 M sulfuric acid. The proposed electrode can generate a current density up to 400 mA cm⁻² and withstand 168 h electrolysis under 10 mA cm⁻² with an initial overpotential of 598 mV. This stability is largely superior to most of the reported noble metal-free OER catalysts, and the activity has been improved >70 mV compared to the other reported Sb-based electrodes. The 1 week electrochemical stability in acidic media was thoroughly studied by XPS and XRD to analyze the chemical composition and morphology of the electrode before and after the OER, respectively.