I'm going strictly on origin when I say it's made with bourbon. It was invented in Louisville by (or in honor or) a bourbon lover:
http://gardenandgun.com/article/old-fashioned
Cocktail lore traces the old-fashioned to late-nineteenth-century Louisville, specifically the Pendennis Club, where the drink was purportedly invented by (or perhaps in honor of) Col. James E. Pepper, a patriarch of Kentucky bourbon whiskey who also kept a successful stable of Thoroughbreds.