Astronotus ocellatus examples have been reported to grow to about 45 cm in length and 1.6 kilograms in weight. The wild-caught forms of the species are typically darkly coloured with yellow-ringed spots or ocelli on the caudal peduncle and on the dorsal fin. These ocelli have been suggested to function to limit fin-nipping by piranha (Serrasalmus spp.), which co-occur with Astronotus ocellatus in its natural environment. The species is also able to rapidly alter its colouration, a trait that facilitates ritualised territorial and combat behaviours amongst conspecifics. Juvenile Oscars have a different colouration from adults, and are striped with white and orange wavy bands and have spotted heads.
Widely distributed in the Amazon region and has been recorded in Colombia, Venezuela, Bolivia, Ecuador, Peru, Brazil, French Guiana, Paraguay, Uruguay and Argentina. Across this range, it has been collected from numerous river systems, including the Ucayali, Solimões, Amazonas, Negro, Madeira, Tapajós, Tocantins, Orinoco, Approuague, and Oyapock. Feral populations also exist in several countries, including Singapore and the USA.
Most often found in the shallows of slow-moving or still waters in forested areas. It appears to favour silt-laden white-water habitats, where it is typically associated with submerged tree roots or under cover of marginal vegetation.
Unsuitable for the careful aquascape aquarium since it has a tendency to dig into the substrate but otherwise relatively unfussy. A natural-style set-up could include a soft, sandy substrate, relatively dim lighting plus some large driftwood roots and branches.
Efficient filtration and a dedicated maintenance regime comprising weekly water changes of 30 – 50% aquarium volume are essential, and if possible the heaters that should be situated externally as this species has been known to break such items of equipment.
This species is a generalised omnivore, feeding on a range of items including smaller fish, insects, crustaceans, zooplankton and various types of both terrestrial and aquatic vegetation in the wild, although meatier foods appear to be the preferred option with fruit and other plant material being taken when prey is scarce.
Although not especially aggressive for a cichlid of this size this species’ predatory nature dictates that it should not be combined with much smaller fishes. Its adult size also precludes the possibility of it being maintained in a community in all but the largest private aquaria. Potential tankmates include characids, anostomids, other cichlids, and larger loricariid or doradid catfishes.
In wild populations, adult males are sometimes a little larger and more colourful than females, but this species does not tend to exhibit external sexual differences. In nuptial females, the ovipositor is visible as an extension to the oviduct just prior to and during spawning.
It is a popular aquarium fish although its adult size and typical lifespan of 10 – 20 years must be taken into account before purchase.