Well, your current set up is flawed. Barbs are tropical fish requiring a temp in the 70's. Goldfish are coldwater fish requiring temp in the 60's. I don't know what temp you are keeping this 55 at but its got to be bad for the fish that aren't getting their natural temp range. Another thing to consider is that barbs are fin nippers and goldfish are slow moving. Most likely your goldies are getting picked on by the barbs.
Now, oscar get to be in the range of 10''-18'' and are agressive. It is true they aren't "as" aggressive as other cichlid but they will no doubt get all your current fish. Some people will argue against keeping 1 oscar in a 55 but I'm not one of those people. If you want to get an oscar you will have to get a different tank for the goldfish and barbs. Even then you will only be about to keep 1 full grown adult oscar in the 55g. That's assuming that your oscar doesn't happen to be on the ranger end of the spectrum and grow to be 18'', in which case you will need a bigger tank or rehome him. A 55gal's width in less than 18'', I believe, and the fish wouldn't be able to turn around. The reason why they need atleast a 55 per oscar is because of the amount of waste they produce. Good luck.
