Electrochemical catalysts for oxygen evolution reaction are a critical component for many renewable energy applications. To improve their catalytic kinetics and mass activity are essential for sus-tainable industrial applications. Here, we report a rare-earth metal-based oxide electrocatalyst comprised of ultrathin amorphous -La2O3 nanosheets hybridized with uniform -La2O3 nanoparticles -(La2O3@NP-NS). Signifi-cantly improved OER performance is observed from the nanosheets with a nanometer-scale thickness. The as-synthesized 2.27-nm -La2O3@NP-NS exhibits excellent catalytic kinetics with an overpotential of 310 mV at 10 mA cm-2, a small Tafel slope of 43.1 mV dec-1, and electrochemical impedance of 38 ?. More importantly, due to the ultrasmall thickness, its mass activity, and tover frequency reach as high as 6666.7 A g-1 and 5.79 s-1, respectively, at an overpotential of 310 mV. Such a high mass activity is more than three orders of magnitude higher than benchmark OER electrocatalysts, such as -IrO2 and -RuO2. This work presents a sustainable approach toward the development of highly efficient electrocatalysts with largely reduced mass loading of precious elements.