| Odontotrichum scabrum Rydb., 1924 |
| Odontotrichum schaffneri (A. Gray) Rydb., 1924 |
| Odontotrichum silphiifolium (B.L. Rob. & Greenm.) Rydb., 1924 |
| Odontotrichum sinuatum (Cerv.) Rydb., 1924 |
| Odontotrichum tridactylitis (B.L. Rob. & Greenm.) Rydb., 1924 |
| Odontotrichum tussilaginoides (Kunth) Rydb., 1924 |
| Oedera L., 1771 |
| Oedera trinervia Spreng., 1800 |
| Ogiera Cass., 1818 |
| Oglifa (Cass.) Cass., 1822 |
| Oglifa californica (Nutt.) Rydb., 1918 |
| Oglifa depressa (L.) Chrtek & Holub, 1963 |
| Oglifa gallica (L.) Chrtek & Holub, 1963 |
| Oliganthes Cass., 1817 |
| Oliganthes corei Cuatrec., 1956 |
| Oliganthes discolor (Kunth) Sch. Bip., 1847 |
| Oliganthes ferruginea Gleason, 1922 |
| Oliganthes karstenii Sch. Bip., 1860 |
| Oliganthes karwinskii Sch. Bip., 1847 |
| Oliganthes oxylepis Benth., 1873 |