You’ll simply pop them in the oven at 200 F. (93 C.) for about 4-6 hours. It all depends on the thickness of your oranges, so after 4 hours, see if they're still a bit wet. You’ll know they’re done when they’re only slightly bendable. If they’re not completely dry, don’t worry, they’ll continue to dry over the next few days on your tree. The best time to make them into ornaments is actually a day or 2 after drying in the oven because the centers might still be a bit wet after they come out. Giving them a day or 2 to further dry out on a countertop really makes sure they're nice and completely dry, but depending on how thinly you sliced your oranges, they might be ready to go right out of the oven.