| Ophryosporus scabrellus (B.L. Rob.) B.L. Rob., 1925 |
| Ophryosporus solidaginoides (Kunth) Hieron., 1900 |
| Orcya Vell., 1825 |
| Oreochrysum Rydb., 1906 |
| Oreochrysum parryi (A. Gray) Rydb., 1906 |
| Oribasia Schreb., 1789 |
| Oribasia acaulis Moc., 1837 |
| Oritrophiinae Nesom, 2020 |
| Oritrophium (Kunth) Cuatrec., 1961 |
| Oritrophium durangense G.L. Nesom, 1998 |
| Oritrophium orizabense G.L. Nesom, 1992 |
| Orsinia Bertol. ex DC., 1836 |
| Orthopappus Gleason, 1906 |
| Orthopappus angustifolius (Sw.) Gleason, 1906 |
| 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 |