Arrange orange sections in a lightly oiled 8- to 10-cup ring mold; set aside.
Sprinkle gelatin over 1/2 cup orange juice in a large bowl; stir and let stand 1 minute.
Bring remaining 1 1/2 cups orange juice and sugar to a boil in a small saucepan, stirring constantly; stir into gelatin mixture. Stir in champagne. Chill until consistency of unbeaten egg white; pour over orange sections in mold. Cover and chill 8 hours.
Unmold onto a serving plate lined with mixed salad greens.
