We report the effect of the concentration of hydroxide ion adsorbed (OHad) on indium tin oxide (ITO) in the electrodeposition of zinc oxide (ZnO) by potential modulation method. We found that by applying an optimal constant potential of -0.72 V, X-ray diffraction (XPD) peak intensity of crystalline phase indicating ZnO (100) and ZnO (101) deposited in oxygen containing solution was significantly higher than that of ZnO formed in oxygen free solution. ZnO was islanding on ITO substrate in an oxygen free solution, while uniform ZnO underwent bulk deposition with higher growth rate due to higher concentration of OHad resulted from dissolved oxygen. Thus, morphological images were in good agreement with the crystal structural analysis. In pulsed potential method, several ZnO peaks in ex situ XRD analysis were obtained on ITO substrate as optimal cathodic potential of -0.72 V for 5 s was superimposed on off-time of 5 s at open-circuit potential in each cycle. When relatively shorter cathodic time of 1 s was applied, however, we did not obtain any XRD peak of ZnO, because it might be due to the lack of the critical concentration of OHad to form ZnO on the surface.