| Carex nigromarginata f. minor Boeckeler, 1881 |
| Carex novogaliciana Reznicek, 1990 |
| Carex oaxacana L.H. Bailey, 1898 |
| Carex occidentalis L.H. Bailey, 1889 |
| Carex oerstedii Liebm., 1850 |
| Carex olivacea Liebm., 1850 |
| Carex oreocharis Holm, 1900 |
| Carex orizabae Liebm., 1850 |
| Carex ovata Burm. f., 1768 |
| Carex oxycarpa Holm, 1905 |
| Carex paneroi Reznicek & S. González, 2021 |
| Carex pellita Muhl. ex Willd., 1805 |
| Carex percostata F.J. Herm., 1950 |
| Carex perlonga Fernald, 1907 |
| Carex perstricta Mack., 1909 |
| Carex pertenuis L.H. Bailey, 1889 |
| Carex peucophila Holm, 1905 |
| Carex phleoides Cav., 1799 |
| Carex phleoides subsp. koyamae (Gómez-Laur.) Jim. Mejías, 2018 |
| Carex physorhyncha Liebm. ex Steud., 1885 |