The dramatic enhancement of the silicon room-temperature oxidation rate by a Na overlayer has been attributed to the spectator catalytic action of the alkali metal. In contrast, we show evidence that Na participates in the oxidation process binding an oxygen atom in a substoichiometric intermediate oxidation state. This causes preferential saturation of the semiconductor dangling bonds on the surface and a subsequent promotion of the oxide growth. We have, in fact, detected the Na 2p photoemission chemical shifts when bonds with an oxygen atom are established in a hybrid configuration.