Hello and welcome. My name is ***** ***** I'm a biologist with a special interest in fish. I suspect the reason no one responded to you sooner is that no one wanted to give you bad news. I just logged on and saw your question. I didn't want you to think you were being ignored, and I believe you deserve honesty. While there are reports of Oscars living to be in their twenties, those fish are the exceptions. The average life expectancy is 10 to 13 years. It is very likely that your fish does not have a treatable illness, but is suffering from old age.
There are really only three choices in this situation. The fish has no symptoms that indicate a common disease, so pet store medicines are not going to help. You could consult an aquatic vet who would be able to help determine what is wrong - kidney failure, heart disease, etc. - and tell you if anything can be done. If you'd like that option, this link will take you to a directory of aquatic vets:
A second option is to let nature take its course until your fish passes.
The third choice is to humanely end his suffering. If you decide to do this, I can give you the most humane method (there's no additional fee). I only give that when someone requests it because some people don't even like to consider it or hear about it.
If you have more questions, let me know in a REPLY. I wish I could give you a home remedy, but unfortunately, this is a situation where there isn't one.
My goal is to provide you with excellent service – if you feel you have gotten anything less, please reply back, I am happy to address follow-up questions. Please remember to rate my service only after you have all the information you need. Thank you!