Ranunculus flabellaris (yellow water buttercup)

Opuntia ficus-indica (Barbary fig)
Opuntia humifusa
Opuntia macrorhiza var. macrorhiza (twistspine pricklypear)
Opuntia phaeacantha (tulip pricklypear)
Opuntia polyacantha (plains pricklypear)
Orbexilum pedunculatum var. pedunculatum (Sampson's snakeroo
Orbexilum stipulatum (largestipule leather-root)
Orchis mascula
Orchis pallens
Orchis ustulata
Ornithogalum umbellatum
Ornithogalum umbellatum (sleepydick)
Orobance minor
Orobanche elatior
Orobanche hederae
Orobanche ramosa (hemp broomrape)
Orobanche uniflora (oneflowered broomrape)
Oryzopsis asperifolia (roughleaf ricegrass)
Osmorhiza depauperata (bluntseed sweetroot)
Osyris alba
Oxalis acetosella
Oxalis corniculata
Oxalis corniculata (creeping woodsorrel)
Oxalis grandis (great yellow woodsorrel)
Oxalis priceae ssp. priceae (tufted yellow woodsorrel)
Oxalis stricta
Oxalis stricta (common yellow oxalis)
Oxalis violacea (violet woodsorrel)
Oxypolis filiformis (water cowbane)

Image of Ranunculus flabellaris (yellow water buttercup) 1
USDA-NRCS PLANTS Database / Britton, N.L., and A. Brown. 1913. An illustrated flora of the northern United States, Canada and the British Possessions. Vol. 2: 105.