| Obolaria virginica | Virginia Pennywort |
| Oenothera biennis | Biennial Evening-primrose |
| Oenothera humifusa | Sea-beach Evening-primrose |
| Oenothera laciniata | Cutleaf Evening-primrose |
| Oenothera parviflora | Small Flowered Evening-primrose |
| Oenothera perennis | Perennial Evening-primrose |
| Onoclea sensibilis | Sensitive Fern |
| Ophioglossum pycnostichum | Southern Adder's-tongue |
| Ophioglossum vulgatum | Southern Adder's-tongue |
| Opuntia humifusa var. humifusa | Eastern Prickly-pear Cactus |
| Orchis spectabilis | Showy Orchis |
| Ornithogalum umbellatum | Star-of-bethlehem |
| Orobanche uniflora | One-flowered Broomrape |
| Orontium aquaticum | Golden Club |
| Orthotrichum ohioense | Ohio Bristle Moss |
| Orthotrichum stellatum | Bald Bristle Moss |
| Osmorhiza claytonii | Bland Sweet-cicely |
| Osmorhiza longistylis | Scented Sweet-cicely |
| Osmunda cinnamomea | Cinnamon Fern |
| Osmunda claytoniana var. claytoniana | Interrupted Fern |
| Osmunda regalis var. spectabilis | Royal Fern |
| Osmundastrum cinnamomeum var. cinnamomeum | Cinnamon Fern |
| Oxalis corniculata | Creeping Woodsorrel |
| Oxalis dillenii | Southern Yellow Woodsorrel |
| Oxalis dillenii subsp. filipes | Southern Yellow Woodsorrel |
| Oxalis florida | Southern Yellow Woodsorrel |
| Oxalis stricta | Upright Yellow Woodsorrel |
| Oxalis violacea | Violet Woodsorrel |