When the British Empire ruled the seas, all trade depended on wooden casks- everything from spices to flour, tobacco to gunpowder, wine and beer were all packed in casks for transport.  Recent years have seen an explosion of American brewers using bourbon or new oak barrels to flavor and age their best brews. The flavor imparted from oak can be used to enhance the overall character of the beer. 

To varying degrees, all the beers below use oak somewhere in the brewing process.