I can't give you an absolutely conclusive answer to why your fish isn't eating, it's very difficult to determine this in these situations. Water quality often plays a part though. Exactly what is your ammonia. There aren't any chemicals that actually remove ammonia as far as I know, they will detoxify it though. For this reason, you will still get a reading of ammonia although it shouldn't be toxic. How long is your nitrate levels stayed at zero? I don't mean to sound patronising but are you absolutely sure that you are doing the test properly. If you don't shake the bottle and vial for long enough on some tests, you'll get a false reading, often zero.
Your Oscar is eating, okay not much but at least he is eating. It's when they completely stopped eating for long periods of time, that's when there could be problems. Just hang in there, get your water sorted out and hopefully everything should be okay:)