| Oglifa californica (Nutt.) Rydb., 1918 |
| Oglifa depressa (L.) Chrtek & Holub, 1963 |
| Oglifa gallica (L.) Chrtek & Holub, 1963 |
| Oliganthes Cass., 1817 |
| Oliganthes karwinskii Sch. Bip., 1847 |
| Oliganthes oxylepis Benth., 1873 |
| Oligogyne DC., 1836 |
| Oligogyne tampicana DC., 1836 |
| Oligosporus Cass., 1817 |
| Oligosporus campestris (L.) Cass., 1826 |
| Oligosporus dracunculus (L.) Poljakov, 1961 |
| Oligosporus glaucus (Pall. ex Willd.) Poljakov, 1961 |
| Oligosporus mexicanus (Willd. ex Spreng.) Less., 1832 |
| Olivaea Sch. Bip., 1872 |
| Olivaea leptocarpa De Jong & Beaman, 1963 |
| Olivaea tricuspis Sch. Bip., 1872 |
| Omalotheca Cass., 1828 |
| Oncosiphon Källersjö, 1988 |
| Oncosiphon pilulifer (L. f.) Källersjö, 1988 |
| Onoserideae Solbrig, 1963 |