The role of oxygen diffusion in the response of individual SnO2 nanowires to this gas is studied. Different oxygen partial pressures lead to strong changes of their electrical resistance, even at room temperature. Since surface models fail to explain the experimentally observed long-term resistance transients, it is necessary to make a description of the interaction mechanisms between oxygen species and SnO2 nanowires by taking ion diffusion into account. Our model correctly describes the experimentally measured dependence of the nanowire resistance with oxygen partial pressure, and it can be applied to the characterization of other metal oxide materials.