It was considered as one of the biggest orchid genera and some authors indicated 750 species and some situated it at about 600. Due to the studies realized, part of sections of the genus has been elevated to generic rank under different nomenclature, so nowadays, the number of species has decreased.

Karleinz Senghas informs (in a revision of Die Orchideen by Rudolf Sclechter, which was published in Schlecher, Die Orchideen, 3^rd. edition, l997) that until now, 910 species have been described under the name Oncidium. He could certified, by his revision, the existence of 315 species, around them turn nothing less than 222 synonyms and the rest of 373 names are, nowadays, located in independent genera.
However Brazil, with aproximately 100 species, is still considered as the center of geographic distribution.

Three publication are very important to understand this genus and the new genera of Subtribus Oncidiniae:
1) "Orchidaceae Brasiliensis (vol. II)", by Pabst and Dungs,
2) "Synopsis of the Genus Oncidium", by Leslie A. Garay and John E. Stacy, published in Brazil, in Bradea, vol I, nº40, l974 and
3) the recent studies by Karlheinz Senghas, above.

The genus occurs in central and tropical America, from Florida to Argentina, however the most past of species occurs in Brazil, Colombia (Andes region), Ecuador and Peru. The species occur from the sea level to high altitudes (4.000m elevation). Some species require a dry climate and some others which come from Atlantic Forrest need a humid environment.

It grows on the tree, rocks or as terrestrial but there are epiphytic species found growing as terrestrial or rupiculous and vice-versa. It is very difficult to separate rupiculous and terrestrial species.

The inflorescence arises every year from a new shoot and produces from one to more than hundreds flowers depending on the species. It appears from the base of the pseudobulbs or from the axils of the leaves.

Some species are known by the size of the inflorescences adn by the great number of flowers. O. altissimum (Central America species) and O. baueri (Brazilian species) with ascendant panicles of 5m. longer or O. sarcodes (relativity small plant) with 3m. longer.

In some species such as Oncidium papilio Lindl e O. kramerianum Rchb. f. (nowadays Psychopsis), the flowers arise from the same spike for years so it should not be cut when a flower fades.
The size of the flowers varies from millimeter to 10cm across. The size of the plants also from the small species of Plurituberculata Section (nowadays Lophiaris) with pseudobulbs with less than 0,50cm tall such as