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| Ocellochloa | rudis, stolonifera |
| Ochagavia | andina, carnea, elegans, litoralis |
| Ochetophila | nana, trinervis |
| Ocimum | americanum, campechianum, carnosum, gratissimum, nudicaule, ovatum |
| Ocotea | aciphylla, acutifolia, bicolor, brachybotra, catharinensis, corymbosa, daphnifolia, densiflora, diospyrifolia, dispersa, elegans, floribunda, glaziovii, gracilipes, indecora, lanata, lancifolia, laxa, lobbii, marumbiensis, minarum, nectandrifolia, notata, nunesiana, nutans, odorifera, paranaensis, porosa, porphyria, prolifera, puberula, pulchella, silvestris, spectabilis, suaveolens, teleiandra, tristis, urbaniana, vaccinioides, velloziana, velutina |
| Octomeria | alexandri, anceps, chamaeleptotes, chloidophylla, concolor, crassifolia, cucullata, decumbens, diaphana, fibrifera, flabellifera, gracilis var. gracilis, gracilis var. pauloensis, grandiflora, imigiae, irroarata, juncifolia, leptophylla, lichenicola, liliputana, linearifolia, lithophila, micrantha, ochroleuca, octomeriantha, palmyrabellae, praestans, pusilla, rigida, robusta, rodriguezii, rohrii, rotundiglossa, stellaris, tricolor, unguiculata, warmingii |
| Ocyroe | armata |
| Odonellia | eriocephala |
| Odontocarya | acuparata, asarifolia, tamoides var. canescens |
| Odontonema | cuspidatum |
| Odontorrhynchus | castillonii, chlorops |
| Oeceoclades | maculata |
| Oedochloa | minarum, procurrens |
| Oenanthe | crocata, globulosa |
| Oenothera | acaulis, affinis, arequipensis, bahia-blancae, biennis, catharinensis, centaurifolia, coquimbensis, curtiflora, elongata, glazioviana, grisea, indecora, laciniata, lasiocarpa, longiflora ssp. longiflora, longiflora ssp. grandiflora, longituba, magellanica, mendocinensis, mollissima, montevidensis, nana, odorata, parodiana, pedunculifolia, peruana, picensis ssp. cordobensis, picensis ssp. bonariensis, picensis ssp. picensis, punae, ravenii ssp. ravenii, ravenii ssp. chilensis, rivadaviae, rosea, sandiana, santarii, scabra, siambonensis, stricta ssp. stricta, stricta ssp. altissima, suffrutescens, tafiensis ssp. parviflora, tafiensis ssp. tafiensis, versicolor, villaricae, villosa |
| Oldenlandia | brachypetala, corymbosa, dusenii, lancifolia, salzmannii |
| Oleandra | australis |
| Olfersia | cervina |
| Oligocladus | patagonicus |
| Olsynium | acaule, biflorum, bodenbenderi, chrysochromum, filifolium, frigidum, junceum ssp. junceum, junceum ssp. colchaguense, junceum ssp. depauperatum, lyckholmi, nicolae, nigricans, obscurum, philippii ssp. illapelinum, philippii ssp. philippii, scirpoideum ssp. luridum, scirpoideum ssp. scirpoideum, scirpoideum ssp. leucanthum, scirpoideum ssp. scirpeum, trinerve |
| Olyra | ciliatifolia, fasciculata, glaberrima, humilis, latifolia |
| Ombrophytum | chilensis, subterraneum |
| Omphalodes | linifolia |
| Oncidium | emaculatum, fuchsii, maculosum, verrucosissimum |
| Oncorachis | macrantha |
| Onira | unguiculata |
| Ononis | mitissima |
| Onopordum | acanthium, nervosum |
| Onoseris | alata, hastata |
| Onuris | alismatifolia, graminifolia, hatcheriana, papillosa, spegazziniana |
| Oocephalus | oppositiflorus |
| Operculina | hamiltonii |
| Ophioderma | palmata |
| Ophioglossum | crotalophoroides, fernandezianum, melipillense, nudicaule, opacum, reticulatum, vulgatum |
| Ophiopogon | japonicus |
| Ophryosporus | anomalus, axilliflorus, burkartii, charua, heptanthus, johnstonii, laxiflorus, lorentzii, macrodon, paradoxus, pinifolius, piquerioides, regnellii, triangularis |
| Ophthalmoblapton | crassipes |
| Oplismenopsis | najada |
| Oplismenus | hirtellus ssp. hirtellus, hirtellus ssp. setarius |
| Oplonia | jujuyensis |
| Opuntia | anacantha var. anacantha, anacantha var. kiska-loro, anacantha var. retrorsa, anacantha var. utkilio, arechavaletae, assumptionis, aurantiaca, bonaerensis, canterae, colubrina, discolor, elata var. cardiosperma, elata var. elata, elata var. obovata, ficus-indica f. ficus-indica, ficus-indica f. amyclaea, megapotamica var. megapotamica, megapotamica var. salagria, megapotamica var. chadihuensis, monacantha, penicilligera, quimilo, robusta, salmiana, spinulifera, stenarthra, sulphurea var. hildemanii, sulphurea var. pampeana, sulphurea var. sulphurea, ventanensis |
| Oreobolus | obtusangulus |
| Oreocereus | celsianus, hempelianus, leucotrichus, trollii |
| Oreopanax | capitatus, fulvus, kuntzei |
| Oreopolus | glacialis |
| Oriastrum | acerosum var. acerosum, acerosum var. dasycarpum, achenohirsutum, apiculatum, chilense, dioicum, gnaphalioides, lycopodioides, minutum, pentacaenoides, polymallum, pulvinatum, pusillum, revolutum, sphaeroidale, stuebelii var. abbreviatum, stuebelii var. argentinum, tarapacensis, werdermannii |
| Origanum | x majoricum |
| Orites | myrtoideus |
| Ormosia | arborea, fastigiata |
| Ornithocephalus | brachystachyus, myrticola |
| Ornithopus | compressus, micranthus, pinnatus, sativus |
| Orobanche | minor, ramosa, tarapacana |
| Ortachne | breviseta, rariflora |
| Orthochilus | ruwenzoriensis |
| Orthoclada | laxa |
| Orthopappus | angustifolius |
| Orthosia | congesta ssp. brachystephana, congesta, dusenii, florida, guilleminiana, loandensis, paranaensis, retinaculata, scoparia, subulata, teodormeyeri, tomentosa, urceolata, virgata |
| Orthrosanthus | occissapungus |
| Oryza | latifolia, rufipogon, sativa |
| Osmia | ramosissima |
| Osmorhiza | berteroi, depauperata, glabrata, mexicana |
| Osmunda | spectabilis |
| Osmundastrum | cinnamomeum |
| Ossaea | flaccida |
| Osvaldoa | valida |
| Otachyrium | versicolor |
| Otholobium | glandulosum, higuerilla, pubescens |
| Ottelia | brasiliensis |
| Ouratea | claudei, multiflora, paraguayensis, parviflora, salicifolia, sellowii, spectabilis, vaccinioides |
| Ourisia | alpina, breviflora ssp. breviflora, breviflora ssp. uniflora, coccinea ssp. coccinea, coccinea ssp. elegans, fragrans, fuegiana, microphylla, muscosa, polyantha, pulchella, pygmaea, ruellioides, serpyllifolia |
| Ovidia | andina, pillo-pillo |
| Oxalis | adenophylla, alstonii, arbuscula, arenaria, argentina, articulata ssp. articulata, articulata ssp. rubra, atacamensis, aureoflava, barrelieri, benjaminii, bifrons ssp. bifrons, bipartita ssp. bipartita, bipartita ssp. pabstii, bisecta, bisfracta, brasiliensis, bulbocastanum, caesia, calachaccensis, chachahuensis, cinerea, clandestina, colchaguensis, compacta, confertissima, conorrhiza, corniculata var. atropurpurea, corniculata var. corniculata, cotagaitensis, cratensis var. cratensis, cratensis var. paraguayensis, cytisoides, debilis, dumetorum, enneaphylla, eremobia, ericoides, eriocarpa, eriolepis, erosa f. erosa, erosa f. rosea, erythrorhiza, famatinae ssp. famatinae, famatinae ssp. sanjuaninae, floribunda ssp. ostenii, frutescens, geralensis, gigantea, grisea var. mattogrossensis, gyrorhiza, hedysarifolia, hepatica, hispidula, holosericea, hyalotricha, hypsophila, incana, johnstonii, laciniata, lasiopetala, latifolia ssp. latifolia, latifolia ssp. schraderiana, laxa, leucophylla, linarantha, lindneri, loricata, lourteigiana, magellanica, matancillae, megalorrhiza, mira, monticola, morenoensis, morronei, muscoides, myriophylla, nahuelhuapiensis, niederleiniana, niederleinii, novemfoliolata, nubigena, odonellii, ornithopus, ovalleana, pachyrrhiza, paludosa, pampeana, paranaensis, pardoensis, penicillata, perdicaria, pes-caprae, pinguiculacea, platylepis, potamophila, praetecta, pseudoviolacea, psoraleoides ssp. insipida, purpurea, pycnophylla, ranchillos, refracta, renifolia, rhombeo-ovata ssp. rhombeo-ovata, rhombeo-ovata ssp. sustenta, ricardii, riparia, rosea, rupestris, sarmentosa, sellowiana f. sellowiana, sellowiana f. alba, sellowii var. rosea, sellowii var. sellowii, sellowii var. alba, serpens var. serpens, serpens var. catharinensis, sleumeri, spiralis ssp. membranifolia, squamata, squarrosa, strictula, subacaulis, subvillosa, tacorensis, telmatica, tenerrima, tortuosa, triangularis ssp. papilionacea, triangularis ssp. triangularis, trollii, tuberosa, valdiviensis, virgosa, yacutulensis |
| Oxossia | capitata |
| Oxybasis | ambigua, frigida, glauca, halophila, macrosperma, macrosperma ssp. salsa, parodii, urbica |
| Oxycaryum | cubense f. paraguayense |
| Oxychloe | andina, bisexualis, castellanosii, haumaniana, mendocina |
| Oxypetalum | acerosum, aequaliflorum, alpinum var. pallidum, appendiculatum, arenicola, argentinum, arnottianum, aurantiacum, balansae, banksii, barberoanum, brachystemma, brachystephanum, brussae, capitatum ssp. capitatum, capitatum ssp. angustatum, capitatum ssp. mirabile, ceratostemma, charrua, chodatianum, coalitum, coccineum, coeruleum, commersonianum, confusum, cordifolium ssp. pedicellatum, corymbosum, crispum, dusenii, erectum, erianthum, fiebrigii, filamentosum, foliosum, fontellae, glabrescens, globosum, gracile, guaraniticum, hilarianum, hoehnei, humile, incanum, insigne var. insigne, insigne var. bresolinii, insigne var. burchellii, insigne var. glabrum, joergensenii, kassneri, kleinii, lineare, longipedunculatum, lynchianum, macrolepis, malmei, marchesii, marginatum, marianae, martii var. martii, martii var. nanum, megapotamicum, microphyllum, molle, morilloanum, mosenii, muticum, nigricans, obtusifolium, ophiuroideum, ostenii, pachyglossum, pachygynum, pannosum var. pannosum, pannosum var. macranthum, parviflorum var. parviflorum, parviflorum var. kuhlmannii, pubescens, radinsii, reflexum, reitzii, rojasianum, schulzii, solanoides, stipatum, subcapitatum, sublanatum, suboppositum, sylvestre, teyucuarense, timbense, tomentosum var. parvifolium, tomentosum var. tomentosum, tubatum, tucumanense, venustum, warmingii, wightianum |
| Oxyphyllum | ulicinum |
| Oxytheca | dendroidea ssp. chilensis |
| Oziroe | acaulis, argentinensis, arida, biflora, pomensis |
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