The oxidation of CO adlayers, formed by direct dosing from a GO-saturated solution, and bulk CO has been studied on Pt[n(111) x (111)] single crystals in 0.5 M H2SO4. For the stepped Pt surfaces studied, CO is found to adsorb preferentially on the steps, blocking the electrochemical hydrogen adsorption there. A pronounced effect of electrode surface structure on CO oxidation has been observed. The overpotential for the oxidation of a saturated CO adlayer, as well as of submonolayer CO coverages, is found to increase in the sequence Pt(553) < Pt(554) < Pt(111), i.e, with decreasing step density. Interestingly, at low CO coverages the peak potential of CO oxidation does not depend on the CO coverage for Pt(553) and Pt(554), while it increases linearly with the CO coverage in case of Pt(111). This indicates that at low coverages CO does not block surface sites for water oxidation on stepped surfaces. Bulk CO oxidation is also found to be sensitive to the step density. Similarly to CO adlayer oxidation, the onset potential of dissolved CO oxidation increases in the sequence Pt(553) < Pt(554) < Pt(111). The catalytic effect of steps is explained in terms of the enhanced formation of oxygen-containing species, needed for CO oxidation, at surfaces with a higher step density.