It all depends on the tank size really, a single oscar on its own with nothing else is going to need to be housed in a 55g. When adding other fish there is lots to be considered do you have the space, the filtration capacity, the temperament of the fish as larger Cichlids can be a real handful. My tank works because they are in 6ftx2ftx2ft and they have plenty of space.A good way to start keeping Oscar is getting a 75g - 90g and having a shoaling fish like Silver Dollars or Giant danios and one of the convicts which should work in a tank that size IMO. The real key to having sucess keeping them would be by doing as much research on the fish as you can.