75 Gallons MINIMUM for one oscar.

They grow extremely fast. A lot of places recommend a 55 gal for one oscar...but to me that doesn't make sense. A 55 gal tank is what, 12 3/4" deep? Oscars grow to about a 12" in a year, and that would mean for an O to turn around 360 degrees, it would have less than 1/2" of clearance on either side...not good.

They are really cool fish though. They are the type of fish that's way more concerned about whats going on outside of their tank than anything else. They'll beg for food, definitely grow to recognize you.

We have an O and a 2 severum in 75 gallon in the reception area of the office I work in. Seems like he "greets" every person that walks by.

As far as aggression...totally depends on the individual fish. Some get along in groups, others don't. Unless you have a very large tank, as you said, I would stick with one. If you want to have a few tankmates, make sure you get them all at about the same size...Oscars generally are not very aggressive, they just use their size to intimidate other fish.

I say thumbs up to an O, they are great fish.

Hope that helps.