Okay, you've come to the right place for information on Oscars. Firstly, did you cycle the tank before putting the Oscar in? The next thing you must do is test the water for ammonia, nitrite, nitrate and pH. You will need to get some test kit to do this, to get all of them will probably cost to an excess of £10, $20. However, they are extremely important to your fishes well fair.
The next thing I would ask you to do is provide me with a photograph of your fish so that I can ascertain how bad your fishes condition is. You obviously newcomers to keeping Oscars so you do have a lot to learn and stop I would suggest that you read everything that is on this website, it will tell you everything you need to know regarding keeping Oscars. There is a section on feeding, setting the tank up, health, everything. To be quite honest, it is essential reading for any newcomer.
I can't really help you any more until I know what the water conditions are in your tank.
I would also ask you to look carefully at your is it, is it breathing heavily at all, i.e. opening and closing its mouth rapidly ? If it is, please carry out a 30% water change and make sure you use a water conditioner to remove harmful elements such as chlorine and chloride.
