Surface roughening of Si under bombardment with oblique O2+ beams at energies between 0.5 and 2 keV was studied with atomic force microscopy and secondary ion mass spectrometry. At beam energies of 1 keV and below, the general features of the topography and the magnitude of the roughness depended critically on the incidence angle. In most cases there were two angular ranges where surface roughening was strong, in-between and at grazing incidence roughening was minimal. Apart from the known topographical features-ripples and irregular bumps-triangular elevations were observed. In many cases, the local angle of incidence at the beam-facing slopes of the ripples corresponded to the maximum in sputter rate. Furthermore, the average distance between adjacent ripples increased with depth until saturation. It is concluded that the complexity in the topographies is caused by a delicate balance between several roughening and smoothing mechanisms.