Grate oranges to equal 2 teaspoons grated rind; set rind aside.
Peel oranges. Remove and discard white pith; separate into sections. Place orange sections in a heatproof serving bowl; set aside.
Cook sugar, 1/4 cup water, and corn syrup in a heavy saucepan over medium heat, stirring often until sugar dissolves.
Increase heat to medium high, and bring mixture to a boil. Boil, without stirring, 8 to 12 minutes or until syrup is a pale golden-amber color. Carefully stir in orange juice, lemon juice, and orange rind. (Mixture will harden.) Cook, stirring constantly, 3 minutes or until mixture returns to liquid consistency.
Remove from heat, and immediately pour hot syrup over orange sections. Let cool to room temperature. Stir in liqueur. Cover and chill 8 hours. Gently stir oranges before serving.