Courtesy: Huntington Long Island NY
Cut oranges in 1/2 and hollow out (don't want any pulp left at all. Get it all the way to the white-ish part)
Set hollowed orange halves, open side up, in something secure so they don't move around when the jello is in them (I used muffin pans; easy transportation, great to keep them where you want 'em =D)
Prepare jello with 1/3 less water needed for recipe. Pour into o...range halves
Set in fridge and chill completely
Take out of fridge, flip over onto cutting board (open side down so it's flat on the surface). Cut into slices, either halves again (1/4 of the whole) or into 3 slices. Make sure to use a really good serrated knife, and don't be really forceful with them, it's jello after all! =)
And voila! You have jello orange slices!