Line a rimmed baking sheet with parchment paper, and place a wire rack on top; spray rack with cooking spray.
Arrange orange slices in a single layer, ½ inch apart, on rack.
In a large saucepan, bring sugar, 3 cups water, and corn syrup to a boil over medium-high heat. Cook until sugar is dissolved and temperature registers 325° on a candy thermometer, 13 to 15 minutes. Slowly pour caramel over oranges. Let cool until hardened.
Using a knife, carefully remove oranges from rack. Refrigerate, in a single layer, in an airtight container. Use within a few hours, as fresh oranges will weep.
Recipe by Victoria at https://victoriamag.com/favorite-fall-recipes/