We investigate the effects of the geometrical structure (phase and porosity) of multilayer benzene films on the desorption of O- induced by 2-20 eV electron impact on varying quantities of absorbed O-2. Differences in the yield of O- from O-2 doped amorphous and crystalline benzene films are attributed to the ability of O-2 to diffuse into the amorphous solid via pores and defects formed during its deposition at 20 K. In contrast, diffusion into crystalline benzene is limited and deposited O-2 molecules remain at the surface of the film. Thermal desorption measurements support this analysis. The data are also compared with results of similar experiments for O-2 on water. While it is apparent that some of the variation in O- yield observed from ice films is similarly related to morphology, a substantial suppression of the O- yield is likely to result from energy loss by electrons prior to dissociation. Quenching of intermediate O-2(-) states by water ice may also contribute to this suppression in the range 5-12 eV.