Engineering single-atom electrocatalysts with high-loading amount holds great promise in energy conversion and storage application. Herein, we report a facile and economical approach to achieve an unprecedented high loading of single Ir atoms, up to similar to 18(wt)%, on the nickel oxide (NiO) matrix as the electrocatalyst for oxygen evolution reaction (OER). It exhibits an overpotential of 215 mV at 10 mA cm(-2) and a remarkable OER current density in alkaline electrolyte, surpassing NiO and IrO2 by 57 times and 46 times at 1.49 V vs RHE, respectively. Systematic characterizations, including X-ray absorption spectroscopy and aberration-corrected Z-contrast imaging, demonstrate that the Ir atoms are atomically dispersed at the outermost surface of NiO and are stabilized by covalent Ir-O bonding, which induces the isolated Ir atoms to form a favorable similar to 4+ oxidation state. Density functional theory calculations reveal that the substituted single Ir atom not only serves as the active site for OER but also activates the surface reactivity of NiO, which thus leads to the dramatically improved OER performance. This synthesis method of developing high-loading single-atom catalysts can be extended to other single-atom catalysts and paves the way for industrial applications of single-atom catalysts.