| 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 |
| Othake roseum Bush, 1904 |
| Otopappus Benth., 1873 |
| Otopappus acuminatus S. Watson, 1891 |
| Otopappus aschenbornii Klatt, 1894 |
| Otopappus asperulus S.F. Blake, 1922 |
| Otopappus brevipes B.L. Rob., 1909 |