Zest the oranges and the lime and add all of the zest to a medium sauce pan. Now juice the oranges, adding the juice to the pot. You need 2 cups fresh orange juice. Add the sugar.
Bring the mixture to a boil over medium heat. Once at a boil, reduce the heat to a simmer and cook for about 8 to 10 minutes, or until the syrup has reduced by about 1/3. Remove the syrup from the stove and let cool slightly. Pour the syrup through a fine mesh strainer into a glass jar. Discard the pulp and zest from the strainer and place the jar in the fridge until the liquid is cold, about one hour.
When you're ready for a drink, mix one part syrup and two parts seltzer in a glass filled with ice. Stir it up and serve with an orange slice if desired. Enjoy!