When you say you have got a big tank, how big are we talking? Six Oscars will require over 300 gallons. Remember we are talking about fish that can reach in excess of 12 inches. So there is a possibility that you could have 6 feet of fish, not including the rest of their body mass, that will be a huge bioload. You will need the big tank to cope with that and you will need extremely good filtration as well.
If you were to have six Oscars together, I can't tell you whether your Oscars will get cranky, it's impossible to answer that question. The law of averages say that you are bound to have at least one male and female that could pair up and start breeding. You could well have a couple of breeding pairs. Problems could start arising if you do end up with either one, or two breeding pairs. They might not be too happy about other fish being in the tank when they have eggs. Fish and Cichlid in general can get very aggressive when they start breeding.
