Blot the orange slices with paper towels to remove moisture.
Place the slices in a single layer on top of the parchment paper.
Bake in the oven (120C/250F) for around 3 hours. Check hourly to make sure the oranges are not burning. If they are starting to brown but not dried yet, reduce the temperature of the oven and keep checking until fully dry.
Notes
Store in an airtight container. They are edible for up to 2 years and are great for decorations for as long as you want!