Preheat the oven to 200°F and line a baking sheet with parchment paper.
Using a mandoline or sharp serrated knife, slice the oranges very thinly (about ⅛ to ¼ inch). The thinner the slices, the quicker they will dry out!
Gently blot with a paper towel to remove excess moisture.
Arrange on the baking sheet with a little bit of space between each slice.
Bake at 200°F for 2-4 hours, flipping every 30 minutes to make sure the orange slices don't burn.
Flipping the slices also gives you the chance to check how dry they are. Some slices will dry more quickly than others so go ahead and remove them. [This is especially important to keep in mind if you're drying various types/sizes of citrus!]
If the slices begin to brown but don't feel dry, that's a sign your oven is too hot!
Remove from the oven and let cool completely to room temperature before storing.
Notes
Store in an airtight container in a cool, dark place for up to 2 years.