OMO has been in the hobby a long time but around 2000 Paul Loiselle showed the form of Pachypanchax from the type locality of turned out it was misidentified as OMO in the mid 20th century proved to be P. arnoulti which was a new species. This is the true OMO from the type locality of Nosy Be island, a tiny island off the west coast of Madagascar. Previously it was thought the species could be found up and down the west coast but these turns out to be distinct species. Perhaps the biggest shock was the discovery that there are multiple color forms of the fish which occur in the same place at the same time.
Photos of OMO in literature prior to 2006 will be arnoulti not omolonotus.
P. omalonotus can be told apart from P. arnoulti by the presence of metallic gold spots on the flanks, absence of iridescent white edges to the anal fin and lower caudal fin lobe and less rounded dorsal and anal fins.