Oxygen evolution reaction (OER)-induced reconstruction on precatalysts generally results in surface-reconstructed catalysts with less active species and thus low mass activity. Herein, the deeply reconstructed (DR) catalyst is proposed and derived from a sub-10 nm precatalyst to achieve high-mass-activity catalysis. As a proof-of-concept, the DR-NiOOH with a multilevel nanosheet structure interconnected by sub-5 nm nanoparticles was obtained via a lithiation-induced deep reconstruction strategy. The robust DR-NiOOH with abundant active species enables its significantly enhanced mass activity (170 mV decrease in OER overpotential to achieve 5 mA mg^–1) and better durability (>10 days) than that of incompletely reconstructed Ni@NiOOH. Its strong corrosion resistance (30 wt % KOH, 72 h) and high thermal stability (52.8 °C, >40 h) were also confirmed. Theoretical analyses support that the unsaturated OH coverages on orthorhombic NiOOH endow its good OER-active property. This work highlights the merits of high-utilization DR catalysts toward potential catalytic applications under realistic conditions.