In this study, the initial step of the oxygen evolution reaction (OER) under alkaline conditions is theoretically investigated for a Co-based isonicotinate metal-organic framework (MOF). To construct a computationally feasible model of the periodic MOF structure, monomer, tetramer, pentamer, and extended cluster models are considered. The extended cluster model, which includes a central Co site, the surrounding MOF environment, and the hydrogen-bonding network, is used for subsequent calculations. OH- adsorption is then investigated at the central Co, pyridine, carboxylate, and hydrogen-bonding network sites through geometry optimization. The optimized structures and relative energies show clear differences depending on the adsorption site, with the hydrogen-bonding network site exhibiting the lowest relative energy. In contrast, the central Co and carboxylate sites show relatively high energies, accompanied by dissociation of a Co-coordinated H2O molecule. These results indicate that the initial OH- adsorption behavior in the Co-based isonicotinate MOF is influenced by the local structural environment and provide a theoretical basis for further investigation of the OER pathway.