Pour the bourbon, simple syrup, and bitters into a 1-gallon drink dispenser.
Add the cold water and stir for about 20 seconds to combine.
Cover and refrigerate the dispenser for at least 1 hour and up to 3 days.
Give the dispenser one slow stir right before serving, since the ingredients can settle.
Place a single large ice cube in each rocks glass.
Pour the mixture from the spigot over the ice, filling each glass about two-thirds full.
Twist a strip of orange peel over each glass to release its oil, then drop in a brandied cherry.
Good to Know
For the zero proof dispenser, replace the bourbon with 2 cups strong black tea, cooled completely, and keep everything else the same; the dash of bitters per glass carries too little alcohol to matter.