We demonstrated a high-yield and easily reproducible synthesis of a highly active oxygen evolution reaction (OER) catalyst, "the core-oxidized amorphous cobalt phosphide nanostructures". The rational formation of such core-oxidized amorphous cobalt phosphide nanostructures was accomplished by homogenization, drying, and annealing of a cobalt(II) acetate and sodium hypophosphite mixture taken in the weight ratio of 1:10 in an open atmosphere. Electrocatalytic studies were carried out on the same mixture and in comparison with commercial catalysts, viz., Co3O4-Sigma, NiO-Sigma, and RuO2-Sigma, have shown that our catalyst is superior to all three commercial catalysts in terms of having very low overpotential (287 mV at 10 mA cm-2), lower Tafel slope (0.070 V dec-1), good stability upon constant potential electrolysis, and accelerated degradation tests along with a significantly higher mass activity of 300 A g-1 at an overpotential of 360 mV. The synergism between the amorphous CoxPy shell with the Co3O4 core is attributed to the observed enhancement in the OER performance of our catalyst. Moreover, detailed literature has revealed that our catalyst is superior to most of the earlier reports.