The effect of oxygen on the photovoltaic properties of organic solar cells consisting of zinc phthalocyanine and a perylene pigment has been investigated. Under solar illumination, it was possible to raise the short-circuit current by a factor of 1.5 when increasing the partial O-2 pressure threefold from atmospheric pressure in a surrounding gas atmosphere consisting of different oxygen:nitrogen ratios with a total pressure of 1 bar. On the other hand, the exciton diffusion length in ZnPc was observed to decrease at higher oxygen pressures, from which we conclude that the range of exciton transport in the ZnPc layer is limited by the presence of ionic impurities such as O-2(-).