In a medium skillet, combine the water and sugar and bring to a boil. Add the orange slices and cook over moderate heat, turning them occasionally, until the liquid is reduced to a thin syrup and the orange slices are translucent, about 20 minutes.
Reduce the heat to moderately low and simmer until the syrup is thick and the slices are tender but still intact, turning occasionally, about 10 minutes.
Transfer the orange slices to a rack to cool. Reserve the syrup for another use.