Whatever that curiosity you drank may purport to be, it is certainly not an Old Fashioned. Citrus is a rarity in a stirred cocktail at the best of times and certainly not something you would expect to find in an OF!
Originally the OF was meant to be a good spirit dressed up a bit with sugar, bitters, citrus peel and nothing more. An OF is the easiest cocktail you could make, but the most complicated to perfect.
Fortunately there is an
IBA official guide to start with, but
this is an excellent 'standard' recipe from a superb YouTuber.
You can pretty much 'Old Fashion' anything (the original was likely made with gin) just stick with the basic recipe and tweak your amounts.
Spirit
Sweetener
Bitters
Ice
If you like rum, try a Tiki Old Fashioned.
2oz Rum (Appleton Estate Signature is a decent starting point)
0.5oz Orgeat Syrup
Dash of Bittermens Tiki Bitters / Whatever your preference is.
Stir with ice, then strain into a chilled rocks glass with fresh ice.
This is a good twist recipe, if you like Tequila and Mezcal.