Developing highly active oxygen evolution reaction (OER) electrocatalysts is crucial and highly desirable to improve the efficiency of water electrolysis for H-2 fuel generation. In this work, hierarchical MoO42- intercalating alpha-Co(OH)(2) nanosheet assemblies were synthesized from the readily available metal source and 2-methylimidazole as the alkaline reagent by green and fast coprecipitation. The ultrathin alpha-Co(OH)(2)-MoO4(25) nanosheets possess a lateral size of about 40-100 nm and a thickness from 2.5 to 4.6 nm (3 to 6 layers). Triggered by OER oxidation treatment, the alpha-Co(OH)(2)-MoO4(25) material undergoes ultrafast and thorough structure reconstruction to CoOOH(25) by only one linear sweep voltammetry scan and chronopotentiometry within 35 s. The in situ transformation leads to the formation of the CoOOH active phase with rich oxygen vacancies and distorted structure, extracting MoO42- ions at the same time. Compared with other designed counterparts, CoOOH(25) offers a lower overpotential (309 mV) for the OER at a current density of 10 mA cm(-2) and a smaller Tafel slope of only 57.78 mV dec(-1), representing an excellent unary CoOOH-based or Co(OH)(2)-based catalyst. The improved catalytic activity of CoOOH(25) can be attributed to its specific structures, e.g., ultrathin disordered nanosheets that could facilitate electrolyte transportation and expose more active sites, oxygen vacancies that enhance the intrinsic per-site activity, and their synergistic effect. This established structure self-renewal technology might also be expanded to other advanced nanomaterials toward other electrochemical energy conversion processes.