1. Pour 2 cups juice into a large bowl. Sprinkle gelatin over and let stand for 5 minutes.
2. In a saucepan, combine 2 cups remaining juice with sugar. Place over medium-low heat, stirring until sugar has dissolved, and bring to a simmer, about 7 minutes. Pour warmed juice over gelatin mixture and stir until gelatin has dissolved. Stir in remaining 2 cups juice.
3. Lightly mist a 10-cup Bundt pan with cooking spray. Pour in 1 cup juice mixture and refrigerate until set, about 25 minutes. Scatter half of oranges on top and pour in 2 more cups juice mixture. Refrigerate until set, about 35 minutes. Repeat with remaining oranges and juice mixture. Refrigerate until cold and completely set, at least 4 hours.
4. When ready to serve, gently pull gelatin mold away from edges of pan and invert onto a serving plate.