Yes, aluminum reacts to O2, but mostly very superficially, it actually “overcorredes” (sorry, I don’t know the English), creating a thin layer of protection against water. That’s why aluminum is considered a good material corrosion wise (you will see that window frames are usually made of aluminum instead of other metals for this reason, among others).
Aluminium corrosion resistance is very good in untreated aluminium. Untreated aluminium has very good corrosion resistance in most environments. This is primarily because aluminium spontaneously forms a thin but effective oxide layer that prevents further oxidation.
http://www.aluminiumdesign.net/design-support/aluminium-corrosion-resistance/
Probably the problem is more a mechanical than chemical. Probably the combination of abrasion and salty sweat have sanded the handlebar.
But again aluminum is a material well regarded for standing corrosion where other materials would just rust to nothing, like iron.