I agree with making an effort to find out what he was fed by his previous owner. He must have been eating something to get that big! Plenty of fish are fed a single food for so long that they become "addicted" to it and may become picky about switching, but Oscars are generally very easy feeders that will always eat if conditions are right. An Oscar hunger-strike is usually pretty short.
Why did you put him into a 50-gallon shoebox, with little room to move and very unstable water conditions? That move certainly isn't an ideal step towards improving his appetite. I understand that there may be emergency situations that call for a temporary move into a very small enclosure, but in those cases the best course is to reduce or eliminate feeding altogether for a short period of time to preserve water quality...which of course is not an issue with a well-fed fish, but can be hard on a fish that is not eating to begin with.
If his water and tank conditions are indeed good...and that's a big "if" because it sure sounds like something is messing with him...then I would suggest getting some live nightcrawlers from a bait shop. I consider them to be just about the most irresistible food for any carnivorous fish; if they don't get him eating, then there is something truly wrong with his circumstances and you need to figure out what it is.