Whilst you can keep a single Oscar in a 55g (4ft, 14"wide) tank, it doesnt allow it a great deal of space, particulary considering they can reach 14-18", imagine trying to turn in that space.
So, obviously the larger the tank the better, personally a 75g (4ft, 18" MINIMUM) is what I would say is the smallest you should keep a single Oscar in.
Buying two young does not guarantee they will be friends for life, in a lot of cases, once they reach maturity, they will fight for dominance (and territory) and this can leave either one or both seriously injured. There is no way of sexing them until a pair forms, so buying two in the hope they will pair up is not a good idea. Whilst two from young can and does work, you have to be prepared for the fact that it wont. From your other thread, the 40g tank is not large enough for even one.
You obviously need good filtration with an Oscar as they are extremely messy fish, so plenty of water changes too, they also require a good staple diet along with live/frozen foods (not as some people think feeder fish as these offer no nutrional value) and if the tank is large enough you can house some large dither fish, like silver dollars, tinfoil barbs, bala sharks etc but these need to be large when introduced so they are not seen as food, an Oscar will consider anything it can fit in its mouth as food. They grow large quickly, with good food and water conditions, they can grow an inch + a month until they reach 8", where upon they slow slightly until they reach full size, so obviously they have large appetites and large mouths!
