Hey Sack
Great to see you're interested in Oscars. They are a great fish with lots of personality. Keep in mind, any juveniles you buy for your 55 will need a bigger tanks as they grow older. Of course, that’s if you become attached to them. They are a game fish in South America, very abundant and plenty of them. Watch people flame me for that one;-). That said (more than once) they will thrive in a wide range of water condition compared to some fish. Here are some general guidelines.
Get a good filter!! Oversized for your tank if you can. Ensure allot of aeration occurs from the filter or add a stone and pump. Oscars diet consists of high protein, which eventually ends up as nitrATE (NO3). The only way to effectively remove this is to do water changes. The effect of this build up will be minimized between water changes if the fish have an ample supply of oxygen.
Don't use live plants. Oscars love to rearrange things. A good trick it to buy attractive artificial plants, remove the base, drill a hole in a lava rock and epoxy the plant to the rock. These will still look attractive when the fish pull them up and they won't float.
Oscars get ick!! Just have some meds on hand. It is a combination of the polluted water and live food.
Oscars will eat almost any kind of general fish food. There are plenty of cichlid pellets available and most have a picture of an Oscar on them. Vary the diet for good nutrition. I like live feeders, that’s part of the charm of Oscars but they can introduce diseases.
Maintain a stable pH any were from 6.6 to7.4, key word is "stable"
Good luck and have fun!!!