| Ormosia panamensis Benth. ex Seem., 1853 |
| Ormosia schippii Pierce ex Standl. & Steyerm., 1946 |
| Ormosia toledoana Standl., 1935 |
| Ormosia velutina Rudd, 1960 |
| Orobus L., 1753 |
| Orobus arborescens Sessé & Moc., 1889 |
| Orobus argenteus Mill., 1759 |
| Orobus diffusus Nutt., 1813 |
| Orobus dissitifolius Alef., 1861 |
| Orobus faba Brot., 1804 |
| Orobus polymorphus (Nutt.) Alef., 1861 |
| Orobus sericeus Sessé & Moc., 1890 |
| Orobus trifoliatus Sessé & Moc., 1894 |
| Otholobium C.H. Stirt., 1981 |
| Otholobium sericeum (Poir.) C.H. Stirt., 1986 |
| Ottleya Sokoloff, 1999 |
| Ottleya greenei (Wooten & Standl.) D.D. Sokoloff, 1999 |
| Ottleya mollis (A. Heller) D.D. Sokoloff & Gandhi, 2003 |
| Ottleya nivea (S. Watson) D.D. Sokoloff, 1999 |
| Ottleya oroboides (Kunth) D.D. Sokoloff, 1999 |