In saucepan, boil sugar and 2 cups/500 mL water until slightly thickened, about 5 minutes. Meanwhile, cut oranges into 1/4-inch/5 mm slices; submerge in syrup and simmer until rind is translucent, 8 to 10 minutes.

With slotted spatula or spoon, lay oranges on parchment paper-lined baking trays; bake in 300°F/150°C oven until dried, 15 to 20 minutes. Carefully lift oranges to loosen; let cool on tray on rack. Dry overnight or longer, until thoroughly dried. Store in airtight container until ready to hang on tree.

Makes about 30 orange slices.