Developing efficient oxygen evolution reaction (OER) electrocatalysts is important for enhancing the water splitting efficiency. However, with the current catalysts containing one kind of active sites, it is challenging to achieve low overpotentials because of the four-electron transfer process. Herein is reported HZIF-2-CoMo, a new metal-organic framework with well-defined Co-Mo dual sites that can promote the OER process through an unconventional Mo-6(+)/Co-2(+) dual-site relay mechanism. Theoretical calculations suggested that the Mo and Co sites stabilize the HO* and HOO* intermediates, respectively, and that the unique Co-O-Mo configuration induces the formation of a Co-O*-Mo transition intermediate, remarkably reducing the reaction free energy. As a result, HZIF-2-CoMo shows an overpotential of 277 mV at 10 mA cm(-2) and a low Tafel slope of 70 mV dec(-1) in alkaline solution, making it one of the best OER electrocatalysts reported to date.