Highly active, stable, and cut-price (photo-)electrocatalysts are desired to overwhelm high energy barriers for anodic oxygen evolution reaction processes. Herein, a heterostructure of cobalt-iron oxide/black phosphorus nanosheets is in-situ synthesized via a facile and novel three-electrode electrolysis method. Bulky black phosphorus is exfoliated into its nanosheets at the cathode while the CoFe oxide is derived directly from the metal wire anode during the electrolysis process. This heterostructure exhibits excellent electrocatalytic oxygen evolution reaction (OER) performance, and the overpotential at 10 mA·cm⁻² is 51 mV lower than that of the commercial RuO₂ catalyst. Its superior OER performance stems from the favorable adsorption behavior and an enlarged electrochemical active surface area of the catalyst. To reveal the origin of excellent OER performance from the point of adsorption strength of OH*, methanol oxidation reaction (MOR) test is applied under the identified OER operating conditions. Further introduction of light illumination enhances the OER activity of this heterostructure. The overpotential drops down to 280 mV, benefiting from pronounced photochemical response of black phosphorus nanosheets and iron oxide inside the heterostructure. This work develops a new electrochemical method to construct high performance and light-sensitive heterostructures from black phosphorus nanosheets for the OER.