Safe tank mates for Oscar fish include other cichlids like black convicts, firemouths, Texas cichlids and chocolate cichlids; small fish such as silver dollars, danios, tetras and barbs; and certain species of scavenger catfish. Some Oscars are voraciously aggressive, while others are more likely to tolerate their tank mates if they are well-fed.
Cichlids are fairly large and aggressive fish, and their ability to co-exist with Oscars has to do with their tendency to either confront them or escape from them. Introduce smaller fish such as tetras and barbs into an Oscar tank in schools. Their safety lies in their speed, as they are usually able to out-swim and out-maneuver the slow-moving Oscar. Because of their unusual body shapes and nocturnal habits, catfish are not usually attacked by Oscars. They are useful in scavenging food that Oscars leave behind.
Oscars tend to be large aquarium fish, about 18 inches long and 3.5 pounds in weight. They are also territorial, so an important factor in adding tank mates is the size of the tank. It is not advisable to add fish of other species to an Oscar tank smaller than 95 to 125 gallons. The tank also has to have enough hiding places in case the Oscar gives chase and its tank mate needs to seek shelter.