Buoyancy disorders are quite common in twin-tailed varieties of goldfish (this includes the Oranda). Normally they float, but they can also sink. Fish are normally slightly negatively buoyant and this allows them to swim with ease. Being sunken is not as big an issue as floating. The main reasons are that they won’t be in danger of drying, and do not have the stress of struggling to keep upright. Floating fish can be corrected by removing excess gas from the swimbladder, or by surgical placement of a ballast in their abdomen. On the otherhand, a sunken fish is more difficult to fix. It may require inserting air into the swimbladder (which may be difficult to locate if deflated). You’ll have to maintain the gravel clean and smooth, so that it doesn’t get infections on its ventral surfaces. See also, http://youtu.be/ita7URiO9Qs
However, we must try to find the reason for the issue. Common reasons include bacterial infection (including Mycobacteria), neoplasia (cancer), inflammatory conditions of the swimbladder and cystic kidneys. You’d need to take your fish to see an aquatic veterinarian to make the diagnosis. They may make a blood smear for examination for signs of inflammation, X-ray to find any masses that’s causing improper swimbladder function, fine needle aspirate to see what the mass is (if present), etc.
To learn more about this condition, see also