| Common Name | Latin Name |
|---|---|
| Ohio Spiderwort | Tradescantia ohiensis |
| Oldfield Cinquefoil | see Sulfur Cinquefoil |
| Oldfield Toadflax | see Common Toadflax |
| Orange Puccoon | see Hoary Puccoon |
| Orchardgrass | Dactylis glomerata |
| Oregon Vetch | see Common Vetch |
| Oriental Mustard | see Indian Mustard |
| Osage Orange | Maclura pomifera |
| Oxeye Daisy | Chrysanthemum leucanthemum |
