In a medium saucepan over medium heat, combine sugar, water and orange zest. Stir until sugar has dissolved. Remove from heat.
Stir in Angostura and orange bitters. (Omit this step for plain simple syrup).
Let cool completely. Transfer to a glass jar and seal tightly with a lid. Store in the refrigerator for up to 1 month.
Old-Fashioned Mocktail
In a mixing glass, add the syrup. Top with tea, whiskey alternative and/or bourbon whiskey (per the ingredients listed — see notes).
Add ice and use a bar spoon to gently stir the mixture until chilled.
Strain into a rocks glass with a large ice cube. Top with bitters (if using) and garnish with a cocktail cherry and a piece of orange zest.
Notes
No old-fashioned syrup? Traditional bitters contain alcohol, so if you prefer not to make the syrup, you can use ¼ ounce plain simple syrup and add a few dashes of bitters to the finished drink. To make plain syrup, follow the old-fashioned syrup recipe above, but don't add the bitters into the mix. No zero-proof whiskey? Instead of zero-proof whiskey, you can use 2 ounces of the black tea. However, it's best to combine two ingredients to experience the layers of flavor you'd normally expect in an old-fashioned. Low-ABV version: If you are looking for a less potent cocktail that's not quite a mocktail, you can make the old-fashioned with less alcohol. Replace half the whiskey with either zero-proof whiskey or black tea. Nutrition facts (below) are for a mocktail made with old-fashioned syrup.