We investigated the surface morphology and magnetic properties of Co films grown on MgO (100) substrates before and after treatment with a NaOH solution. The surface morphology evolves from a continuous film into isolated hexagonal nanodisks resulting from the formation of cobalt hydroxide. In the as-grown Co films the magnetic anisotropy behaves as a superposition of fourfold magnetocrystalline anisotropy and uniaxial magnetic anisotropy (UMA), and the magnetization reversal proceeds by two-step domain-wall motion. After the NaOH treatment the UMA is reduced and the magnetization reversal appears to be dominated by magnetization rotation. In addition, the exchange bias effect, which in the as-deposited films results from the presence of a cobalt oxide layer, disappears after the NaOH treatment, and can be reinduced by annealing the sample. The observed significant changes in the magnetic properties can be related to the disappearance and reappearance of a cobalt oxide top layer.