Pour the orange juice in a pitcher larger enough to contain about 8 cups of liquid.
Add sugar and stir to dissolve it.
Add the water to the pitcher and mix well.
Adjust sugar to taste if needed.
Serve in glasses with ice or place the pitcher in the fridge until the water is cold to your liking.
Notes
Depending on how sweet your oranges are, you might need a bit more sugar, so adjust the sweet to your taste. And remember that you can also use other types of sweeteners such as honey, stevia, monk fruit, etc.