Starlight, if you are stuck with a 5 gallon tank for an Oranda you are fighting a true battle indeed. I hear that you are already aware this is a problem, so you have a few options to make the best life possible for this fish in the meantime.
1. You can purchase a large rubbermaid/sterilite/tupperware etc container, they are dirt cheap (and you can get them clear too)! And you can house him in this while you save for a more appropriate tank (40+ gallons). The fish does not need a heater (Unless your house is regularly less than 65 degrees), so all you would need is a filter that would work for your rubbermaid container. You start with the filter you have on the 5 gallon, but get a bigger one ASAP. So that should be your next "purchase goal." Get a filter that will eventually go on the big tank you want to buy
Don't forget to toss his airstone in there
You can move all of the gravel and decos over as well, which will keep it cycled (provided it was cycled originally, right now your tank can't keep up with the bioload).
2. While you think about doing what I just said do a 50% water change daily for this fish. If you have not changed the water in a while (more than a week) then start smaller at 20% and work up to 50% (like do a 20% today, 30% tomorrow 40% the next day then 50%). This will help immensely with your current ammonia problems. You would only need to gravel vac once a week unless you wanted to do it more often. If you do this you have some hope of saving the fish. You can go back to weekly water changes once you have him in a larger rubbermaid type container. This will solve your most immediate dilemma.
3. This is the most humane option: Take your fish back and buy another Oranda when you can house it appropriately.
Goodluck with your fish. I hope that everything works out well! Keep your eyes open for good tanks that are large and cheap. I picked up a 100 gallon tank and solid oak stand and canopy for 100 bucks. It was in a guys front yard and they were moving and didn't want to move the tank. I know guppyart got like 10 free 10 gallon tanks. Things like this happen all the time! All you have to do is keep your eyes and ears peeled, and tell your friends to do the same! (It was actually my friend in the car with me who saw the tank I got).