| Osbertia Greene, 1895 |
| Osbertia bartlettii (S.F. Blake) G.L. Nesom, 1991 |
| Osbertia chihuahuana B.L. Turner & S.D. Sundb., 1986 |
| Osbertia rupicola Rzed. & Zamudio, 2017 |
| Osbertia stolonifera (DC.) Greene, 1895 |
| Osmadenia Nutt., 1841 |
| Osmadenia tenella Nutt., 1841 |
| Osmia Sch. Bip., 1866 |
| Osmia odorata (L.) Sch. Bip., 1866 |
| Osteospermum L., 1753 |
| Osteospermum uvedalia L., 1753 |
| Oswalda Cass., 1829 |
| Oteiza La Llave, 1832 |
| Oteiza acuminata La Llave, 1832 |
| Oteiza mixtecana Villaseñor & Panero, 1993 |
| Oteiza ruacophila (Donn. Sm.) J.J. Fay, 1975 |
| Oteiza scandens Panero & Villaseñor, 1993 |
| Othake Raf., 1836 |
| Othake callosum (Nutt.) Bush, 1904 |
| Othake robustum Rydb., 1914 |