The design and synthesis of efficient electrocatalysts are important for electrochemical conversion technologies. The oxygen evolution reaction (OER) is a key process in such conversions, having applications in water splitting and metal–air batteries. Here, we report ultrathin metal–organic frameworks (MOFs) as promising electrocatalysts for the OER in alkaline conditions. Our as-prepared ultrathin NiCo bimetal–organic framework nanosheets on glassy-carbon electrodes require an overpotential of 250 mV to achieve a current density of 10 mA cm−2. When the MOF nanosheets are loaded on copper foam, this decreases to 189 mV. We propose that the surface atoms in the ultrathin MOF sheets are coordinatively unsaturated—that is, they have open sites for adsorption—as evidenced by a suite of measurements, including X-ray spectroscopy and density-functional theory calculations. The findings suggest that the coordinatively unsaturated metal atoms are the dominating active centres and the coupling effect between Ni and Co metals is crucial for tuning the electrocatalytic activity.