1 oscar in a 55 will do great for well over a year, i've had a pair for almost seven years now in one and they've done great. they've spawned countless times and don't have any signs of hith.
i agree, it is definitely cramped, and i know just because it has worked for me doesn't mean it will work for everyone.
also hith is a huge prob with oscars because they will literally eat themselves to death so people tend to overfeed them and since fish have inefficient digestive systems the overfed oscar excretes lots of partially digested food, which pollutes the water terribly.
as for not being able to keep an oscar in a 55 for six months, thats just a sales ploy, a well looked after, non-overfed oscar, shouldn't have any trouble in a 55 for six months. also hith doesn't killfish as far as i know, it attacks the fish's lateral lin network, so essentially it just makes the fish def. i had a pike cichlid with it's entire lateral line eroded, including its entire face leaving its eyes out on stocks, and he was king of the tank.
the pic of him is only a few months into the disease.
anyway, in my opinion an oscar will do fine in a 55 as long as he is treated well, but bigger is always better.