Substantial variations in the erosion rate during low energy (5.5-10.5 keV) O-2(+) sputtering of polycrystalline Co thin films are demonstrated by crater depth measurements as a function of sputtering time. At normal incidence the erosion rate is constant, while for oblique angles (less than or equal to 55 degrees, with respect to surface normal) the rate is between similar to 30% and 75% higher at the surface compared to that in the "bulk." A clear relationship between the width of the preequilibrium region and the sputtering-induced roughness is shown, as revealed by surface stylus profilometry and atomic force microscopy measurements. Sputtering by Ar+ ions, where no chemical compounds form, results in significantly smoother craters and the erosion rate is constant. The development of the preequilibrium region during O-2(+) sputtering of Co is attributed to oxidation-induced roughening of Co. A model is proposed for the oxidation-enhanced roughening, which is further supported by results obtained from Rutherford backscattering spectrometry measurements of oxygen incorporation during O-2(+) bombardment and Ar+ sputtering combined with oxygen flooding.