| Oplismenus pseudoundulatifolius (Roem. & Schult.) Kunth, 1829 |
| Oplismenus rariflorus J. Presl, 1830 |
| Oplismenus repens J. Presl, 1830 |
| Oplismenus sabulicolus (Nees) Kunth, 1833 |
| Oplismenus setarius (Lam.) Roem. & Schult., 1817 |
| Oplismenus tenuis J. Presl & C. Presl, 1830 |
| Oplismenus thiebautii E. Fourn., 1886 |
| Oplismenus undulatifolius (Ard.) Roem. & Schult., 1817 |
| Oplismenus velutinus (G. Mey.) Schult., 1824 |
| Oplismenus zelayensis Kunth, 1815 |
| Orcuttia Vasey, 1886 |
| Orcuttia californica Vasey, 1886 |
| Orcuttia californica var. californica Vasey, 1886 |
| Orcuttia fragilis Swallen, 1944 |
| Orcuttiinae P.M. Peterson & Columbus, 2007 |
| Oreopoa Gand., 1891 |
| Ornithospermum Dumoulin, 1782 |
| Orrhopygium Á. Löve, 1982 |
| Ortachne Nees ex Steud., 1854 |
| Ortachne floridana (Chapm.) Nash, 1903 |