I've put together a tankmate key to make matching other fish with your oscar easier. The key works like this. On the left are a list of potential Oscar tank mates by species. Each species has a star-rating. The rating is based upon the premise that the two fish are to be kept in the smallest recommendable size tank together at, or until they reach adulthood. Directly behind the rating is the minimum recommended size tank the two can be kept in.
This means 55 gallons for one single adult fish
It’s a thrill when oscar fish breed for the first time. If you’ve been keeping oscars a long time, odds are pretty good that you’ve raised a breeding pair. If you don’t have large adults, you may be able to find some at your local fish store. Oscar fish are often brought back to the fish store after the 1-inch juvenile they purchased grew too big for their aquariums or bullied the other fish in the aquarium. So, if you find adults at the store, how do you pick out fish that are a possible breeding pair?
Oscar Fish Tank Setup - Installing the Correct Size Aquarium
I'll get straight to the point, is your aquarium big enough for one Oscar, plus some tankmates? You should already know that 55 gallons is the absolute minimum for one adult Oscar. If your aquarium does not meet this requirement then you have two choices, either stop reading now and enjoy what you've already got, or upgrade your aquarium by at least 75% and then come back and read the rest of the page. Sorry if that sounds a little bit harsh, but that really is your only two options. Most of the fish I am going to recommend as tank mates will require a minimum of 50 gallons just for themselves, not including the Oscar.
One adult Oscar must have at least 55 gallons of water to itself