Carl is right on here. I ferment with no air lock all the time and oxidation is not one of the issues I have run into. Fruit flies, sure, but while fermentation is active they can't really get in real easy and after it slows down, just crimp the foil tighter. when things are done, pop a rubber band around the foil and it's more or less air tight.
also, a stopper and airlock are cheap and also work well particularly if you have the devil flies in your fermentation area or near by.