| Oenothera tubifera Ser., 1828 |
| Oenothera virgata Ruiz & Pav., 1802 |
| Oenothera wigginsii W.E. Klein, 1962 |
| Oenothera xenogaura W.L. Wagner & Hoch, 2007 |
| Oligospermum (Micheli) H. Hara, 1953 |
| Onagra Mill., 1754 |
| Onagraceae Juss., 1789 |
| Onagreae Dumort., 1827 |
| Onagroideae Eaton, 1836 |
| Oocarpon Micheli, 1874 |
| Oocarpon torulosum (Arn.) Urb., 1931 |
| Pachylophus (Spach) Walp., 1843 |
| Pelozia Rose, 1909 |
| Pelozia (Rose) Plitmann, P.H. Raven & Breedlove, 1973 |
| Pelozia clavata (Brandegee) Rose, 1909 |
| Pseudolopezia Rose, 1909 |
| Pseudolopezia longiflora (Decne.) Rose, 1909 |
| Pterocaulon Ramamoorthy, 1980 |
| Quelusia (Vand.) DC., 1828 |
| Raimannia Rose ex Britton & A. Br., 1913 |