As this post indicates, oscars (Astronotus ocellatus) are held in fairly high regard by me, banner-worthy in fact. I've owned two as pets (thus far) and I have fond memories of them eating a Siamese fighting fish, breaking heaters, flopping around outside the tank, getting scared by earthworms and so forth. Having owned goldfish previously, I was amazed that fish could be so active, intelligent and destructive.
A. ocellatus from the Wikipedia commons
A. ocellatus is a member of the enormous family Cichlidae and is a member of the Neotropical radiation comprising over 407 species (Musilova et al. 2008). The subfamily Astronotinae is regarded as being a sister group to most of the Neotropical radiation (Lopez-Fernandez et al. 2005) and is composed of only three genera. Unfortunately, it doesn't seem that any species aside from A. ocellatus is very well known. Chaetobranchus consists of two species specialized for planktivory; C. semifasciatus has a superficially oscar-like appearance and coloration but specimens attributed to C. flavescens look remarkably different (see here). The Guide to South American Cichlidae notes particular resemblance to the genus Acaronia, a member of a different (and more derived) subfamily. I can't help but wonder if Chaetobranchus is paraphyletic or if C. flavescens reverted to a more ancestral coloration and body shape. Chaetobranchopsis is a closely related genus with two species that are even more rarely discussed. Fishbase indicates that C. australis is non-predatory and likely and micro- and planktophagous feeder and C. orbicularis is probably a filter feeder. Unfortunately, I can't talk much more about these species without rampant speculation.
Then there's the other oscar A. crassipinnis which is mainly distinguished by an overall darker coloration, different position of light and dark bars and different scale and fin ray count. Kullander mentions that the syntypes are either from A. ocellatus or some other species. Astronotus ocellatus is distinguished by having multiple ocelli near the dorsal and caudal fin, although this feature does not show up in every specimen (A. crassipinnis and C. semifasciatus also have a single ocelli) and is believed to be anomalous (Winemiller 1990). Since oscars and similar relatives are large, slow-moving fish they have large fins and the ocelli appear to be an eye mimic that dissuades fin-nipping piranhas from feeding on them (Winemiller 1990).
There's a great deal of information on the web about Astronotus ocellatus: ADW is always a good source, Fishbase is a must and there's even a forum.
My lack of output thus far was due to me getting rather tied up on some very long posts which should hopefully make it out sometime this summer. Stay tuned...
Almeida-Val, V.M.F. et al. 2000. Scaling effects on hypoxia tolerance in the Amazon fish Astronotus ocellatus (Perciformes: Cichlidae): contribution of tissue enzyme levels. Comparative Biochemistry and Physiology Part B 125, 219–226.
Chippari-Gomes, A. R. et al. 2005. Metabolic adjustments in two Amazonian cichlids exposed
to hypoxia and anoxia. Comparative Biochemistry and Physiology, Part B 141, 347 – 355
Lopez-Fernandez, Hernan et al. 2005. Molecular phylogeny and evidence for an adaptive radiation of geophagine cichlids from South America (Perciformes: Labroidei). Molecular Phylogenetics and Evolution 34, 227–244
Musilova, Zuzana et al. 2008. Molecular phylogeny and biogeography of the Neotropical cichlid
fish tribe Cichlasomatini (Teleostei: Cichlidae: Cichlasomatinae). Molecular Phylogenetics and Evolution 46, 659–672.
Sloman, Katherine A. et al. 2006. Tribute to R. G. Boutilier: The effect of size on the physiological and behavioural responses of oscar, Astronotus ocellatus, to hypoxia. The Journal of Experimental Biology 209, 1197-1205
Winemiller, Kirk O. 1990. Caudal Eyespots as Deterrents against Fin Predation in the Neoptropical Cichlid Astronotus ocellatus. Copeia, Vol. 1990, No. 3, pp. 665-673