Homoepitaxial growths of ZnO were performed by low pressure metal organic chemical vapor deposition (MOCVD) on both bulk hydrothermal and thin film buffer layers of ZnO grown by pulsed laser deposition on c-sapphire substrates. The surface roughness, photoluminescence, and x-ray diffraction (0002) peak omega rocking-curve line widths for the bulk and thin film substrates were similar prior to MOCVD growth. After MOCVD growth, it was found that the surface morphology, crystallographic quality, and optical properties of the ZnO layers were far superior to those typically observed for heteroepitaxial growth of ZnO layers on c-sapphire by MOCVD. The MOCVD layers were comparable for growths on both the bulk substrate and the thin film buffer layer. Furthermore, no significant impact of carrier gas was observed. The quality of the ZnO layer appeared to be correlated with the surface morphology and the crystalline quality of the substrate rather than being dependent on whether the substrate was in bulk or thin film form.