Whiskey
The most important part of an Old Fashioned.
The Old Fashioned cocktail is a whiskey based drink. Whiskey itself is a distilled alcohol that can be made from a variety of grains, including wheat, barley, rye, corn, etc. There are many different types and brands of whiskey that you can choose to make your Old Fashioned with.
Typically people use bourbon or rye whiskey to make an Old Fashioned.