We report on the atomic structures of bare and metal-coated ZnO O-polar (000 (1) over bar) surfaces as determined by model fitting of surface X-ray diffraction data. O-polar surfaces measured under typical device fabrication conditions are significantly rougher than Zn-polar (0001) surfaces, exhibiting Gaussian-shaped roughness profiles with a width of about 1.5 unit cells, regardless of the existence of a metal over-layer. All samples show a decreased layer distance between the topmost oxygen and zinc layers, consistent with theoretical predictions. Clear differences are observed in the occupations of the topmost oxygen layer in oxidized-metal-coated samples. (C) 2013 Elsevier B.V. All rights reserved.