What type of whiskey is primarily used as the base in an Old Fashioned cocktail?

The Old Fashioned cocktail is traditionally made using bourbon as its primary base. Bourbon is known for its sweeter profile, which complements the bitters, sugar, and citrus elements in the cocktail, creating a harmonious balance of flavors. While variations of the Old Fashioned can be made with other types of whiskey, bourbon is the most classic and widely accepted choice, particularly in the context of this cocktail's history and development.

Although American whiskey refers to a broader category that includes bourbon, rye, and other styles, it does not specify the preferred type for this specific cocktail. Therefore, bourbon is considered the most accurate answer to the question regarding the base whiskey for an Old Fashioned.