In a medium saucepan, bring the 4 cups of water to a boil.

Once boiling, remove the pan from the heat. Add the black tea bags (or loose leaf tea in an infuser), cinnamon sticks, and the zest of one orange. Allow the tea to steep for exactly 5 minutes to prevent bitterness.

While the tea is steeping, combine the sugar and 1/2 cup of warm water (taken from the freshly steeped tea or reserved) in a separate small bowl. Stir until the sugar is completely dissolved, creating a simple syrup.

Strain the steeped tea mixture through a fine-mesh sieve into a pitcher, discarding the tea bags, cinnamon sticks, and zest. Stir in the fresh orange juice and the sugar syrup.

Add the remaining 4 cups of cold water to the pitcher to dilute the concentrate to your preferred strength. Taste and add more sweetener if necessary.

Chill the iced tea in the refrigerator for at least 1 hour, or until completely cold. Serve over plenty of ice, garnished with a fresh slice of orange.