In a large pot, bring nine cups of water to a boil.
Slice oranges into thin whole slices. Once water is boiling add orange slices and simmer for ten minutes. Fill a large bowl with cold water and ice. Drain and transfer orange slices to bowl with ice water.
In the same pot combine six cups of water, sugar and corn syrup. Cook over medium heat, stirring constantly until sugar is dissolved. Once sugar has dissolved add chilled orange slices and bring to a boil. Reduce heat and boil gently for 45 minutes. Gently stir every 15 minutes to ensure an even coating on all slices.
Carefully remove orange slices and place on parchment lined baking sheets. Let slices dry in a warm place for 24 hours.
Coat slices in sugar and store in a covered container in the refrigerator for up to 1 month.
If desired, dip orange slices in melted chocolate. Place on a baking sheet lined with wax paper and let cool for 15 minutes or until set.