“Wrong” is a pretty hard word. Tonic in an Old Fashioned is definitely outside the norms, though. The classic way to make an Old Fashioned was to put some sugar in the bottom of the glass, add the bitters, and often a splash of soda to help dissolve the sugar (and muddle) before adding the whiskey. I’ve never tried the tonic route, but I would guess it would add a slight additional bitter note the the drink from the quinine.

And the question brings to mind the curious case of the Wisconsin Old Fashioned, typically made with brandy, and often topped with something like 7UP, sour mix, or even lemonade.

The Old Fashioned is such a great model—it leaves a lot of room for experimentation. Thanks for the comment!

I agree—this is certainly one of the greatest of the minimalist cocktails. Thanks for commenting!

The simplest thing would be to just say, “Golly, I don’t have any bitters!” and then proceed without. Cut back the sugar a bit—you’re down to just sweetened whiskey at this point, and it could be a little cloying without the bitters. You’ll be drinking what amounts to the original “sling,” and it’s a good, even simpler, drink. (Oh, yeah—and put bitters on the grocery list.)

A more complicated answer would be to include other spicy or herbal ingredients like absinthe, Benedictine, Campari, and so forth, but there you start to get farther afield from the true Old-Fashioned, along the lines of the Improved Whiskey Old-Fashioned .