Astronotus ocellatus is a species of fish from the cichlid family known under a variety of common names, including oscar, tiger oscar, velvet cichlid, or marble cichlid. In South America, where the species naturally resides, A. ocellatus specimens are often found for sale as a food fish in the local markets. The fish can also be found in other areas, including China, Australia, and the United States. Although its slow growth limits its potential for aquaculture, it is considered a popular aquarium fish.
The species was originally described by Louis Agassiz in 1831 as Lobotes ocellatus, as he mistakenly believed the species was marine; later work assigned the species to the genus Astronotus. The species also has a number of junior synonyms: Acara compressus, Acara hyposticta, Astronotus ocellatus zebra, and Astronotus orbiculatus.
A. ocellatus examples have been reported to grow to about 45 cm (18 in) in length and 1.6 kilograms (3.5 lb) in weight. The wild-caught forms of the species are typically darkly coloured with orange-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 A. ocellatus in its natural environment. The species is also able to rapidly alter its colouration, a trait which 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.