Si wafers cut along (100) or selected vicinal faces were cleaned under ultrahigh vacuum and then exposed to increasing doses of O2 from 1 to 103 L. At each step, the surface was characterized by low-energy electron diffraction, Auger electron spectroscopy, and photoemission yield spectroscopy. The roughness-dependent shape of the dangling bond surface state band is deduced from the latter, and its changes upon O2 adsorption are analyzed.