Weed Herbarium – by Scientific Name
| Oclemena acuminata | whorled aster | Asteraceae (Compositae) - Aster or Sunflower Family |
| Oenothera biennis | common evening primrose | Onagraceae - Evening Primrose Family |
| Ornithogalum umbellatum | star-of-Bethlehem | Liliaceae - Lily Family |
| Osmorhiza aristata | see - Osmorhiza longistilis | |
| Osmorhiza brevistylis | see - Osmorhiza claytoni | |
| Osmorhiza claytoni | sweet cicely | Apiaceae (Umbelliferae) - Carrot Family |
| Osmorhiza longistilis | aniseroot | Apiaceae (Umbelliferae) - Carrot Family |
| Oxalis stricta | yellow woodsorrel | Oxalidaceae - Woodsorrel Family |
| Oxalis corniculata | creeping woodsorrel | Oxalidaceae - Woodsorrel Family |