Oscars are great fish, and you can hold two reasonably well sized ones in a 55 gal tank. The biggest problem that I have had with them is the control of ammonia in the tank, due to the output from them. If you have a couple of big ones, keep a close eye on the ammonia level in the water. Reasonably quick fish can get away from an Oscar, but they can't get away from high ammonia levels, so be careful. Also, they do not do well with live plants. They root them out as soon as they are in the tank. Driftwood and rock formations work better. Make sure that they have good cover, otherwise, they will dig into the gravel to create a shelter, kind of like a Jack Dempsey. Oh, and make sure you have plenty of scum suckers around to take care of the mess. Water changes a plenty. Otherwise, there is no other fish more fun to own.
Having kept several Oscars whilst living at home, when I received a new tank for Christmas I did not hesitate in purchasing a couple of juveniles. The best aquarium fish by far, Oscars have the best personalities, sort of like little dogs. I haven't owned anything over 7-8 inches in length yet, however at this size they were very aggressive and used to like nibbling my fingers when I fed them, and you need to have some pretty good filtering happening. I eventually sold my fish back to the aquarium as I was moving out and the tank was a real hassle to clean, six 6-8 inch Oscars in a 5ft tank tend to make quite a mess. By the way, the only fish I found that Oscars cohabitated with were Gouramis and would you believe Albino Catfish, they ate everything else regardless of size.
I have four albino Oscars one of which is a longfin Oscar. The longfin Oscar is beautiful with long flowing white fins that make him look similar to a large goldfish. I also have seven longfin tiger Oscars that are stunning! These fish are the absolute best fish I have ever owned. One of the Tiger longfins likes to come up to me when I am reading by the tank and he examines the pages of the book. Every time I turn the page he wants to examine the new page, the first time this happened was when I was showing my wife some pictures of Oscars in a book. It was not easy to locate longfin Oscars, I had a pet shop order them for me. Only one pet shop out of the ten I checked had heard of longfin Oscars and was able to get me some. I am now looking to find all of the information possible on how to get them to spawn.
I have a one year old albino oscar, and he has been so much fun to keep. He's sick right now, but he used to do what I called a "hungry dance" every time I walked in my room. I bought him when he was teeny tiny, not knowing too much about oscars, but I have learned tons! I learned a lot of what everyone has said on this page (they will eat anything that fits in their mouths, they will de-root plants, and to watch out when feeding them live fish, because mine has gotten sick from them). I think oscars are a great fish to keep if you want to be entertained, and don't mind the upkeep. =)
I have 5 oscars at home and let me tell ya, they'll eat ANYTHING! The other day my little brother threw 4 fruit loops into the tank and they were demolished in less than 3 seconds! You have to have a really good filtration system because they are major poopers (I have 2 Emperor power filters in my tank) and you shouldn't have any live plants. My oscars are very entertaining and I really recommend them.
I am a proud owner of a 13" Tiger, I learned that sexing these fish is difficult at best but here is some info from a breeder. The female has an enlarged opening roughly the size of a pencil eraser. The male is small like the tip of a pencil. My Oscar is part of the family, for a year we called it Ted now with new knowledge her name is Teddy. Lovely fish. I lost a long fin Albino to a one and a quarter with a twist to the carpet below (a lid is a must and any opening is fair game for acrobatics).