Developing electrocatalysts with rich active sites is highly demanded for an oxygen evolution reaction (OER) with the virtues of high efficiency and cheap cost. Herein, we prepared unique oxygen vacancy-rich core/shell NiO@Co3O4 nanocomposites decorated with Co3O4 nanosheets as efficient OER electrocatalysts. First, NiO@Co3O4 nanocomposites were synthesized through a rationally designed stepwise coprecipitation process followed by controlled heating treatment. With a facile NaBH4 reduction treatment, Co3O4 nanosheets were in situ formed on the surface of NiO@Co3O4 nanocomposites, introducing more oxygen vacancies and edge sites simultaneously. These surface-restructured core/shell NiO@Co3O4 nanocomposites exhibited significantly improved activity as OER electrocatalysts possessing a low overpotential (290 mV at 10 mA cm(-2)) and a small Tafel slope (68 mV dec(-1)) in 1 M KOH media. Our work offers an effective strategy to prepare core/shell metal oxide nanocomposites with adjustable surface structures for electrocatalysis.