Wash and dry the oranges. Using a sharp knife or mandoline, slice the oranges about 1/8 to 1/4 inch thick. Discard the thick edge if it is mostly peel.

Remove any seeds from the slices and gently blot them with a paper towel to remove excess moisture.

Preheat your oven to 200°F. Arrange the orange slices in a single layer on a baking sheet lined with parchment paper or on an oven-safe metal rack.

Bake for 2 to 3 hours if using a metal rack (or 4 to 6 hours on parchment paper). Flip the slices every hour to ensure even drying. Remove them when they are dry and let cool completely before storing in an airtight container.