Like all the other entries, I think the Oscar is great. My first Oscar, a tiger, died this year at the age of 5 1/2 years. It learned alot of tricks and was the best experience I had with cichlids.
Oscars are predatory fish, but keep in mind the only eat what fits in their mouths and do not usually pick on other tank mates. (except for the rare exception, I'd say only 1 in every 100 are this way) The reason that I am saying this is because many people believe that oscars can be kept with Midas Cichlids and Red Devils. This in fact is not the case, I have learned this the hard way. Any other more agressive fish, even if it is half the size of the oscar, should be closely monitored for signs of damage to the oscar. Other than that enjoy these spectacular fish. BTW, they love Krill!!
I had 2 red tiger oscars in a 125 gallon tank with a large pleco. These were the only fish in the tank, and they all grew to be very large over a period of several years. When I finally gave in to my sister's begging and gave them to her, the oscars were 16 inches in length, and the pleco was 14 in. long. I found out by accident one day that if your oscars are hungry, they will eat cat food. My 14 year old son placed a few pellets in the tank as a joke, and before I could get them out, they had been eaten.
Oscars will only eat other fish they can get in their mouth (just like Angels) and they can be sexed fairly easly. I believe that it is the males who have the pointed anal fin and the female's is rounded. I have a pair of tiger oscars and a male red. They breed much like angel's, the female cleans off the slate that is placed at the bottem of the tank (where angel's is placed leaning on the side of the tank). Here is MY opinion on feeding oscars: we feed ours Cichild Gold By Hikkich(sp). Their colors are now brilliant!!! When we bought them they were being fed feeder fish. Thier colors were very dull and they were very unhappy. Now we have them in a somewhat community tank, with kissing gourami and the feeder fish who have grown alot since they are not hungry since they have been eating the cichlid gold pellets.
After reading much of this I just have to add my comments on this as well, I have had Oscars now for 20 years, and at no time have I had less than 12 of them. NO you cannot sex them at all regardless of fin markings, color or anything else, and the only reason you have a problem with ammonia is because a 55 gal tank is not enough room for a pair of adult oscars. Oscars will eat feeder fish, frozen brine, crayfish, pellets, and many other things that are good for them. Stay away from meats such as beef heart and so on, but this and many other things have worked for me and does not mean it will work for you as well.
I agree with Steve Green, I have had oscars for a long time and had them breeding. That's the only way I could tell them apart. They do eat whatever fits in their mouth. I found that out when I came home from work to find my oscar had a 2 inch Frontosa in his mouth. That was the most expensive meal he ever had. They are the most human responsive fish I have ever owned, they follow you all over the tank.