For a start, 5 tanks is a big undertaking. I'd be thinking about if you can handle doing 5 tanks every week, If so. Fair on yourself.
For a oscar you could get away with a 200 litre tank, But I'd only keep one oscar in it and nothing else. These are big fish. Otherwise, Go for a 400 litre tank, can get a couple and a few other fish like a BN plec and such.