We present our observations of SN 2010mb, a Type Ic supernova (SN) lacking spectroscopic signatures of H and He. SN 2010mb has a slowly declining light curve (LC) (åisebox-0.5ex 600 days) that cannot be powered by $^56$Ni/$^56$Co radioactivity, the common energy source for Type Ic SNe. We detect signatures of interaction with hydrogen-free circumstellar material including a blue quasi- continuum and, uniquely, narrow oxygen emission lines that require high densities (i̊sebox-0.5ex 10$^9$ cm$^-3$). From the observed spectra and LC, we estimate that the amount of material involved in the interaction was rs̊ebox-0.5ex 3 M $_☉$. Our observations are in agreement with models of pulsational pair-instability SNe described in the literature.