IMO, and IME
1 O can live OK in a 55, 2 is a crowd. Likewise a full grown Dempsey will make a crowd. They do make good tankmates, and they do like caves and rocks, but make sure you glue eveything together tighly as both fish will wreak havoc on the decor if they can. In the old days i couldn't afford a bigger tank, and kept Oscars in a 55 untill they got about 9 inches and then sold them. I also kept JD's with them, Some JD's are real SOB's others are fairly friendly, depend on the fish whether or not he'll beat up your O. Likewise, some O's will tolerate an aggressive fish, other grow bored with harrassment and kill the offender so they can have a peaceful life again.
In a 55 I would go with 1 Oscar, a pearl gourami, a blue gourami, and a pictus. If your Oscar happens to be a smaller one it may work out OK but be prepared to sell him, or upgrade in a year. Remeber if he can't turn around without bending he probably won't be happy, and yu'll be forever fixing anything in the tank with him. Having kept these fish in a 55 I wouldn't do it again, It's just much nicer in a larger tank but not impossible in a 55 if you know what you are up against. Make sure you filter excessively, and do a lot of water changes. one of the highly suspected causes of hith is long term exposeure to nitrate. I like to keep my O's in less than 20 ppm Nitrate at all times. and haven't seen Hith in any of the fish I have raised myself, only in adoptees.
