In rare cases, alkali metals can have negative O.N.
Say, [Na(2,2,2-crypt)]^(+) Na^(-) contains both "normal" sodium ion (complexed) and sodide ions. This kind of compounds are not very stable, but at least can transiently exist in room conditions.
2012-03-13 07:23:44 補充：
In that case, Na can be a (very weak) oxidizing agent.