For thin orange slices, use a vegetable peeler or knife to remove rind (including some white pith). Cut pieces into whatever thickness you want for your recipes (thinly slice for garnish on top of a cake, or for a candy treat on their own, slice into 1/4-1/2" wide pieces).
In a saucepan, combine water and 2 1/2 cups sugar over medium heat. Stir until dissolved, then allow to boil uncovered for 5 minutes.
Add the orange peel, and reduce to simmer until syrup is reduced to a quarter of its original volume (45-60 minutes). Do not stir at all while simmering, as it may cause large sugar crystals to form in mixture.
Remove peels using a slotted spoon, then place in a shallow dish filled with 1/2 cup of sugar. Toss until all surfaces are coated in sugar.
Shake pieces off of excess sugar and place on a parchment lined baking sheet and bake at 200°F for about 30-60 minutes (check every 20 minutes) or until they are completely dry. Or you can place on a wire rack and allow to dry overnight.
Stored in a dry location, they should keep for weeks.
Notes
Suggestions: dip candied oranges in chocolate, cut up and add to cakes, cookies or bread.