The methods and results of a taxonomic revision of Oberonia in Thailand are presented following a historical outline. The traditional methods and techniques of morphology-based botanical revision work were used, and the revision relied on examination of dried as well as liquid-preserved specimens from 13 herbaria, supplemented with observations of live plants made during 25 field trips to five of the seven floristic regions of Thailand. The taxonomic value of selected morphological characters is surveyed, as are ecological aspects, occurrence patterns in Thailand and the overall geographic affinities of the Oberonia species represented in the Thai flora. Fourty-two species are accepted, whereas the only report of O. latipetala from Thailand is considered dubious. Full taxonomic accounts are provided for the genus and species, including a dichotomous key to the species. Whereas all morphological descriptions have been prepared from Thai material only, the synonymies are intended to be complete; O. acaulis, O. dolichostachys, O. falcata, O. longirachis, O. nitida and O. rasmussenii are proposed as new taxonomic (heterotypic) synonyms. The accepted names O. denticulata, O. hosseusii and O. lotsyana are lectotypified, and solutions are provided for long-standing taxonomic/nomenclatural problems, especially in relation to the name O. iridifolia. All accepted species are illustrated by line drawings, and references are provided to additional good illustrations in the literature.