The effect of O-2 on the reduction of NO over prereduced CaO surfaces is investigated. The experimental results suggest the existence of at least three different reaction channels, of which two are related to the high-temperature reduction of the CaO surfaces and involve the use of extra electrons in breaking the NO bond. The third reaction channel does not employ extra electrons for bond breaking, but the activity is affected by the amount of adsorbed surface oxygens. The difference between the former two reaction channels is found in the temperature needed for an observable activity. The reaction channel which is already active at low temperatures is described by a model based on F-centers, whereas the one which needs elevated temperatures involves a hole transport through the bulk. The activation energy for this transport is determined experimentally using a temperature-programmed reaction technique as well as theoretically by means of ab initio quantum chemistry calculations. Room-temperature exposure to O-2 is suggested to result in a poisoning of the F-centers, but has only a minor effect on the reaction channel proposed for high temperatures. Effects on the reduction of NO of time as well as temperature for the O-2 exposure step are also investigated and found to be consistent with an understanding based on the coexistence of different reaction channels.