i think the general rule seems to be that keeping 2, a male and a female, is ideal, Adding a third doesnt work well unless the tank is huge with plenty of space for 2 to pair off and have their own space. That is unless you just want to keep one, as this is probably the best for most tanks, pretty much any tank under 100 gallons or so. If the tank is big enough it is possibly to keep a bunch in one tank, I've seen big tanks with 6-8 oscars but that was at least a 300 gallon tank if not bigger.
I personally think a 75 gallon tank is big enough for one oscar to be very happy, I dont think you have to double it for 2, 125 is probably enough, but many people would say it is on the low end of the scale.
So I guess if you want to keep 4 oscars, with 2 males and 2 females, your looking at 225 or more? I'm not really sure, that is pretty huge. I think it is safe to say that in all of these different sizes you can have other bottom feeding fish, and possibly even some other decent sized community fish? or less aggressive cichlids?