Click on a common name for a photo of the plant, or click on a letter for names beginning with that letter.
| Oak, Black |
Quercus velutina |
| Oak, Chestnut |
Quercus montana |
| Oak, Dwarf Chinquapin |
Quercus prinoides |
| Oak, White |
Quercus alba |
| Ohio Hair-Cap Moss |
Polytrichum ohioense |
| Old-man-in-the-spring |
Senecio vulgaris |
| Olive, Autumn |
Elaeagnus umbellata |
| Onion, Meadow |
Allium canadense var. canadense |
| Orange, Osage |
Maclura pomifera |
| Orange-grass |
Hypericum gentianoides |
| Orange-grass |
Hypericum gentianoides |
| Orchid, Cranefly |
Tipularia discolor |
| Orchid, Grass-pink |
Calopogon tuberosus var. tuberosus |
| Orchid, Northern Rein |
Platanthera flava var. herbiola |
| Orchid, Purple Fringeless |
Platanthera peramoena |
| Orchid, Small Green Woodland |
Platanthera clavellata |
| Orchid, Three-birds |
Triphora trianthophora |
| Orchis, Green-fringe |
Platanthera lacera |
| Orchis, Showy |
Galearis spectabilis |
| Orchis, White-fringe |
Platanthera blephariglottis |
| Orchis, Yellow-crested |
Platanthera cristata |
| Oregon-grape, Beale's |
Mahonia bealei |
| Ox-eye |
Heliopsis helianthoides var. helianthoides |