Oak barrels are the traditional way to age your wine and add oak flavor during the ageing process. Wines age differently in barrels than in other vessels, largely because of the extractable compounds in oak and the slow but inevitable exposure of wine in barrels to the oxygen in air. The origin of the oak, degree of toast, and the age of the barrel are the most important aspects in barrel selection. New barrels impart very different flavor and aroma signatures than the same barrel after one or two seasons of use.
If you are ageing your wine in a barrel, it is important to "top" the barrel, or completely fill it so as to exclude any air. Maintaining your barrel(s) "topped" and tightly sealed at all times is of the utmost importance. The stopper used to close a barrel is called a "bung" and barrels are "bunged" when they are closed. A tightly bunged barrel will actually form a partial vacuum as water and alcohol diffuse through the wood and evaporate. You should hear a distinct rush of air past the bung as you open your barrel to sample. If you do not, pay attention to the placement of the bung when you close the barrel.
Barrel Maintenance: Preparing your barrel for first time use.