I am sorry I needed to ask questions to be best able to assist you. The tank for all those fish is way too small too small. They should have 15-20 gallons a fish Water quality plays very heavily in how the fish do and I suspect that you had a water quality problem which has caused ammonia burns as well as the bloat you are seeing. This link goes over size of a tank for Oscars: http://www.oscarfishlover.com/tank-setup
I suspect that you are seeing Swim Bladder Disease which affects a fish's swim bladder. This is the organ that helps in maintaining a fish's ability to balance and swim in a normal position. When it becomes blocked up or too full of air it stops functioning properly. This can be a result of water quality, Kidney disease or a Bacterial infection. Poor water quality can play a major issue contributing to the Swim Bladder problem. The first step to helping is to check your water quality. The Ammonia should be 0, Nitrites 0 and the Nitrates 20 ppm or less. You should have an API test kit to check the levels.
We can try some first aid to help the fish.
1) First do a 35% water change and be sure to add some de-chlorinator or use bottle Spring water for replacement. We never do a full water change especially with a sick fish because of the Nitrogen cycling process can be eve harder on the fish.
2) Next please feed a couple of frozen peas in which the shell has been removed. You will need to be sure you crush the peas for the fish. This helps the stool to pass and will sometimes help to relieve the problem.
3) I also suggest using aquarium salt at 1 teaspoon of the salt per gallon of water. The Salt helps to draw out fluids that can cause swelling in the swim bladder disease. The Aquarium salt also will help a natural slime on the skin to help heal the Ammonia Burns.
These are a few of links that give you information about swim bladder disease:
There are some other methods of treating swim bladder which include reducing the amount of water in the tank which helps to decompress the swim bladder, but I would only use that as a last resort.
This link will help you locate Fish Vets if the treatment does not help: http://www.koivet.com/a_fishvets.html If the problem is bacterial or a Kidney issue it can compound the problems.I suggest you get an API
master test kit to test the water asap, because if this is a water quality problem because of over crowding, your other fish can eventually suffer. For the ill fish you will want to pick up some Stress coat to treat the fish. If the Ammonia is over 0, Nitrites over 0 or Nitrates more than 20 ppm you will need to do a 25% water change.
If you changed 100% of the water in the ill fish's tank monitoring the water parameters will be the utmost importance. It will go through a New Tank syndrome you can read about that here https://www.cs.duke.edu/~narten/faq/cycling.html to help this problem, you can add some Nitrifying bacteria to help the cycling, do 25% water changes and test your water. You can also add some rock that has algae on it to help start the tank and aid in the Nitrogen cycle. The whole process in saving the sick fish will be keeping the water quality and parameters correct. He may need an antibiotic, but first stem is stabilizing him in the tank.