Oxygen effect on the electronic structure of the interface between C-60 and highly-oriented pyrolytic graphite was investigated by ultraviolet photoelectron spectroscopy (UPS). The film deposited in ultrahigh vacuum showed downward band bending characteristic of n-type semiconductor, possibly caused by residual impurities working as unintentional n-type dopants. In contrast, the film deposited in O-2 atmosphere (1.3 x 10(-2) Pa) did not show such band bending. Possible origins of this difference are discussed in terms of (i) deactivation of an unidentified dopant, (ii) compensation by O-2 working as an acceptor, and (iii) O-2-related carrier-traps such as O-2 itself, O2 delta-C60 delta+ and/or C60Ox (C) 2007 Elsevier B.V. All rights reserved.