Why do oranda goldfish swim on their backs?

They really shouldn't be swimming like that. This could be a problem with their swim bladder, which often happens with fish, especially goldfish. Also, Orandas are more sensitive because of the bubble-like protrusions on the tops of their heads. When feeding, I would suggest mixing the flakes in with the water (to avoid having your fish gulping in air with their food), or using a sinking pellet food.

It could also be a gas problem. Squishing up a few peas (canned ones are softest, but thawed frozen ones or fresh will work also) in the tank for them to eat can help with this.