Wash and dry the oranges. Slice each orange into slices about ¼" thick using a mandoline or a sharp knife You want the slices to be as similar in thickness as possible so that they all dry at the same rate.
Once you have all of your oranges sliced, place them on a paper towel and gently pat the top to remove some of the juice. The dryer the oranges are, the better they will dehydrate! Place each piece on the dehydrator rack until the entire level is full. Fill all of the levels with your orange slices, making sure to leave room around each slice so that the air can circulate them.
Place the lid on the dehydrator and set it to 135º. It will take anywhere from 6-10 hours for the oranges to fully dehydrate. Start checking them around 6 hours. I found that 8 hours was perfect for my slices.
Store in a sealable container, like a canning jar, in a cool dark place.
Notes
Times will vary depending on how thin the slices are, the climate, the outside and inside temperature at the time, and your dehydrator.
Always read the manual for your dehydrator and follow all safety recommendations.