My oscar is in a 460L with a severum, three silver dollars, a plec, a red tail shark (he was a rescue and would have been eaten if my O wasnt placid) and soon to be a synodontis. Other good companions if your oscar will allow it (some wont allow tank mates) are jack dempsies, firemouths, acaras, raphaels etc. Most new worlds do ok along with the larger cats. Giant danios or silver dollar are sometimes used to liven things up a bit. Oscars arent really the agro fish people think they are so there are a heap of tankmate options if you want to add some others depending on tank size.
Like warren said 400L is really only best for two alone, perhaps a smaller plec, i wouldnt push your luck with big fish that can do damage just incase they decide to turn serial killer on another fish and its not an uncommon thing.
Also i wouldnt attempt to get a breeding pair. If you get a pair you cant have anything else at all in that tank. And if buying juvenilies be aware they do go thru puberty and can get a bit wierd during that time. Anything from digging and moving things to attempting to kill anything in or near the tank.
Oscars do require alot of room, but thats because theyre big messy fish but i wouldnt swap mine for anything. Hes defiantly one of the family and just as interactive as a dog You wont be dissapointed if you do decide to go ahead with them