Oscars have been one of my top favorite Cichlids for over 20 years... A few years back I decided I wanted an "Oscar Pond"... So I bought a 6' round 300 Gal Rubbermaid tub to start it in...
Long story short, it was a flop... I had the Rubbermaid set up for about 5 years and it worked great as a tank... but having multiple Oscars in the same environment didn't work so well...
A group of 6 did fine until they reached 6", then aggression became a major issue amongst them. That was long before any spawning took place. In the end the most I could keep in the 6' round 300 gal tank was 3... two males and one huge female that rotated between the males...
That being said, there were always a bunch of other fish in the tank along with the Oscars... But after trying several different options and a handful of different Oscar specimen... I finally accepted that Adult Oscars would rather not hang out with their own kind unless A) they are a pair or B) They are the weaker species in the tank and group up as a form of self defense.
If you like Oscars as much as I do I think you'll be a lot happier with a pair and a few other species in whatever size tank you decide to go with as opposed to a bunch of quarreling/unhappy/stressed Oscars...