Combine water and sugar in a small pot or skillet over medium heat. Stir until the sugar has dissolved.
Add orange slices and bring to a boil. Reduce heat to low and simmer for 10 minutes, stirring occasionally. Remove from heat and let cool for at least 20 minutes.
Strain the mixture into a bowl to remove the liquid from the orange slices (and any pulp). Discard the orange slices and transfer the simple syrup into a jar or airtight container. Store in the refrigerator.