Oxidization is generally pretty bad for most alcoholic drinks (oxidization is the main component in bottle aging in wine, but much of this has to do with the interaction of the oxygen with tannins and other stuff in the wine - http://www.allbusiness.com/trends-events/trends/11429124-1.html). The oak or whatever wood being used is porous, and this allows some of the alcohol to evaporate (particularly with distilled stuff, wine doesnt spend as much time in the barrel, so it doesn't lose as much in the way of alcohol) . Good stuff does stay in a cask for a long time for just this reason, not only does it pick up more of the good flavor, but the "angel's share" is greater, which mellows the alcohol. New casks are required in America, where it is law that no barrel be used twice, in Europe however, there is no such law, and barrels are used multiple times because this imparts different flavors, which is how you can get a sherry-wood scotch, its literally a scotch aged in a barrel once used for sherry.