Electrocatalytic water splitting using energy from sunlight represents a promising strategy for clean, low-cost, and environmentally friendly production of H2. Unfortunately, the oxygen evolution reaction (OER) at the anode is kinetically slow and represents the bottleneck of this process. Transition metal oxides are good candidates for the anode in electrochemical water splitting. Inspired by recent computational work on β-NiOOH, which is considered the active phase during the charging and discharging process in alkaline batteries, we performed density functional theory calculations with the inclusion of the Hubbard-U correction on selected surfaces of pure and Co-doped β-NiOOH to calculate the energetics of the OER. The goal of the paper is to investigate theoretically whether doping a NiOOH surface with Co might change the mechanism and lower the overpotential of the OER on a specific NiOOH surface, and to what extent the choice of the surface unit cell may affect the results. Our results indicate that the most likely reaction mechanism depends on the amount of Co doping. We find that doping the β-NiOOH surface with only 25% Co decreases the overpotential from 0.28 to 0.18 V. We also find that the theoretical overpotential, and which step is the potential limiting step, depends on the size of the surface unit cell selected in the calculations. This work highlights how optimizing the binding energies of the various intermediates (O, OH and H2O) on the Ni and Co surface sites, may be key to reducing the overpotential.