In a large pot over medium-high heat, stir together water and sugar until dissolved.
Turn heat down to medium-low and add orange slices. Cook at a low simmer for 45 minutes or until oranges are softened.
Remove oranges to a cooling rack to use later (they're deliciously candied now!).
Once syrup has cooled, strain it through a mesh sieve to remove seeds and pulp.
Store in air tight container in the refrigerator for up to 2 weeks.
Notes
Make sure to maintain only a low simmer as a full boil will make the syrup into a taffy-like texture. Check it often to make sure it hasn't reduced too much.
You can use the orange slices as candied oranges for cocktails. Just leave them to dry on the rack until they reach your desired consistency. You can also use them to decorate cakes, to dip in chocolate, or to add to other desserts.