Warm over medium heat, stirring occasionally, until the sugar fully dissolves and the liquid is clear (do not boil vigorously). This takes about 3–5 minutes.

Cool

Remove from heat and let the simple syrup cool to room temperature. Transfer to a clean container and refrigerate if not using immediately. Use 2 teaspoons for the recipe (adjust to taste).

Assemble Orange Soda

Chill glassware

If desired, chill 2 tall glasses in the freezer for 5–10 minutes or fill them with ice now.

Juice oranges

Roll the oranges on the counter to loosen juice, cut in half and juice until you have 1 cup (about 2–3 medium oranges). Strain through a fine mesh if you prefer pulp-free soda.

Combine orange and sweetener

In a measuring cup or small pitcher, mix 1 cup fresh orange juice with 2 teaspoons simple syrup (or agave). Add 1 teaspoon fresh lemon juice if using to brighten the flavor. Taste and increase sweetener by 0.5–1 teaspoon if you prefer sweeter soda.

Add orange flavor boost (optional)

If you want a stronger orange aroma, stir in 2 tablespoons orange soda syrup or 2 tablespoons triple sec (for an alcoholic variation) to the orange mixture. If using triple sec, skip or reduce other sweetener to taste.

Build the drink

Divide ice between the two glasses (about 1 cup ice each). Pour 1/2 cup of the orange juice mixture into each glass.

Top with soda

Slowly pour 2 cups chilled club soda or sparkling water into each glass (total 4 cups), pouring gently to preserve carbonation. Stir once gently with a bar spoon to combine — avoid vigorous stirring.

Garnish and serve

Float an orange slice on each glass and add a mint sprig if desired. Serve immediately with a straw or stirring spoon.