Preheat oven to 175 degrees F or lowest temperature.
Place orange slices on a baking pan lined with a non-stick baking mat. Sprinkle both sides generously with sugar. Place in the oven to dehydrate, leaving oven door slightly open.
Turn slices over every 30 minutes for 2 1/2 to 3 hours as needed, patting liquid with a paper towel and rotating pan.
Allow to cool completely. Store in a glass jar.