Originally Posted by SvilleKid

The more air space you have in a bottle of bourbon, the faster it will oxidize and go down hill. If you don't buy this fact, take your mostly empty bottle of bourbon, and let it sit for 4-6 weeks. Then try it against a fresh opened bottle of the same bourbon. You will quickly throw the old out. That is the reason it is recommended that when you near the end of a bottle of bourbon, either drink it quickly, or have some friends over to help finish it off quickly.

Because of this reaction, I usually only open and finish one bottle of bourbon at a time. And I don't storing it long-term in anything that allows more air space than necessary (this would include a decanter).