Preheat oven to 175 degrees. Line 2 rimmed baking sheets with wire racks. Arrange orange slices over racks in a single even layer. Bake, rotating sheets every 2 hours, until chips are shrunken and dry, 4 to 6 hours. Let cool completely. (They will crisp as they cool.) Store in an airtight container at room temperature up to 1 month.