O - On
| Ocimum basilicum | Basilikum | Sweet Basil | Lamiaceae |
| Oenanthe aquatica | Wasserfenchel Gewöhnlicher | Fine- leaved Water Dropwort | Apiaceae |
| Oenanthe banatica | Wasserfenchel Banat | Banat Water Dropwort | Apiaceae |
| Oenanthe conioides | Wasserfenchel Schierlings | Elbe Water Dropwort | Apiaceae |
| Oenanthe crocata | Wasserfenchel Safran | Hemlock Water Dropwort | Apiaceae |
| Oenanthe fistulosa | Wasserfenchel Röhriger | Tubular Water Dropwort | Apiaceae |
| Oenanthe fluviatilis | Wasserfenchel Fluss | River Water Dropwort | Apiaceae |
| Oenanthe globulosa | Wasserfenchel Kugelfrüchtiger | Round- headed Water Dropwort | Apiaceae |
| Oenanthe lachenalii | Wasserfenchel Lachenal | Lachenal's Water Dropwort | Apiaceae |
| Oenanthe lisae | Wasserfenchel Sardischer | Sardinian Water Dropwort | Apiaceae |
| Oenanthe montis- khortiati | Wasserfenchel Schmalblättriger | Chortiatis Water Dropwort | Apiaceae |
| Oenanthe peucedanifolia | Wasserfenchel Haarstrang | Peucedanum- leaved Water Dropwort | Apiaceae |
| Oenanthe pimpinelloides | Wasserfenchel Bibernell | Corkyfruit Water Dropwort | Apiaceae |
| Oenanthe silaifolia | Wasserfenchel Silgenblättriger | Narrow- leaved Water Dropwort | Apiaceae |
| Oenothera acutifolia | Nachtkerze Spitzblättrige | Acute- leaved Evening Primrose | Onagraceae |
| Oenothera affinis | Nachtkerze Verwandte | Related Evening Primrose | Onagraceae |
| Oenothera ammophila | Nachtkerze Sand | Sand Evening Primrose | Onagraceae |
| Oenothera angustissima | Nachtkerze Schmalblättrige | Narrow- leaved Evening Primrose | Onagraceae |
| Oenothera biennis | Nachtkerze Gewöhnliche | Common Evening Primrose | Onagraceae |
| Oenothera braunii | Nachtkerze Brauns | Braun's Evening Primrose | Onagraceae |
| Oenothera brevispicata | Nachtkerze Kurzährige | Short- spiked Evening Primrose | Onagraceae |
| Oenothera cambrica | Nachtkerze Waliser | Small flowered Evening Primrose | Onagraceae |
| Oenothera canovirens | Nachtkerze Graugrüne | Renner's Evening Primrose | Onagraceae |
| Oenothera casimiri | Nachtkerze König Kasimirs | King Casimir's Evening Primrose | Onagraceae |
| Oenothera clavivera | Nachtkerze Nagelzähnige | Nailed Evening Primrose | Onagraceae |
| Oenothera coloratissima | Nachtkerze Gefärbte | Coloured Evening Primrose | Onagraceae |
| Oenothera compacta | Nachtkerze Dichtfrüchtige | Dense- flowered Primrose | Onagraceae |
| Oenothera coronifera | Nachtkerze Kronen | Crowned Evening Primrose | Onagraceae |
| Oenothera cruciata | Nachtkerze Kreuzblütige | Cross- flowered Evening Primrose | Onagraceae |
| Oenothera deflexa | Nachtkerze Abgebogene | Deflexed Evening Primrose | Onagraceae |
| Oenothera depressa | Nachtkerze Weidenblättrige | Willow- leaved Evening Primrose | Onagraceae |
| Oenothera drawertii | Nachtkerze Drawerts | Drawert's Evening Primrose | Onagraceae |
| Oenothera drummondii | Nachtkerze Strand | Rose Evening Primrose | Onagraceae |
| Oenothera ersteinensis | Nachtkerze Ersteiner | Alsatian Evening Primrose | Onagraceae |
| Oenothera flaemingina | Nachtkerze Fläming | Flaeming Evening Primrose | Onagraceae |
| Oenothera fallacoides | Nachtkerze Irreführende | Misleading Evening Primrose | Onagraceae |
| Oenothera fallax | Nachtkerze Täuschende | Intermediate Evening Primrose | Onagraceae |
| Oenothera fallax f. brevihypanthialis | Nachtkerze Kurzröhrige | Short- tube Evening Primrose | Onagraceae |
| Oenothera fruticosa | Nachtkerze Strauchige | Beach Evening Primrose | Onagraceae |
| Oenothera glazioviana | Nachtkerze Rotkelchige | Red Sepal Evening Primrose | Onagraceae |
| Oenothera gaura | Prachtkerze Zweijährige | Biennial Beeblossom | Onagraceae |
| Oenothera hirsutissima | Nachtkerze Weichhaarige | Soft- hair Evening Primrose | Onagraceae |
| Oenothera hoelscheri | Nachtkerze Hoelschers | Hoelscher's Evening Primrose | Onagraceae |
| Oenothera indecora | Nachtkerze Schmucklose | Unbecoming Evening Primrose | Onagraceae |
| Oenothera indivisa | Nachtkerze Ungeteilte | Undivided Evening Primrose | Onagraceae |
| Oenothera issleri | Nachtkerze Isslers | Issler's Evening Primrose | Onagraceae |
| Oenothera italica | Nachtkerze Italienische | Italian Evening Primrose | Onagraceae |
| Oenothera jueterbogensis | Nachtkerze Jueterboger | Jueterbog Evening Primrose | Onagraceae |
| Oenothera laciniata | Nachtkerze Schlitzblättrige | Cutleaf Evening Primrose | Onagraceae |
| Oenothera lindheimeri | Prachtkerze Lindheimers | Lindheimer's Beeblossom | Onagraceae |
| Oenothera linearifolia | Prachtkerze Verschmälerte | Narrowed Evening Primrose | Onagraceae |
| Oenothera longiflora | Nachtkerze Langblütige | Long- flowered Evening Primrose | Onagraceae |
| Oenothera macrocarpa | Nachtkerze Großfrüchtige | Bigfruit Evening Primrose | Onagraceae |
| Oenothera marinellae | Nachtkerze Marinellas | Marinella's Primrose | Onagraceae |
| Oenothera mediomarchica | Nachtkerze Mittelmärkische | Mark Brandenburg Primrose | Onagraceae |
| Oenothera nuda | Nachtkerze Nackte | Nude Evening Primrose | Onagraceae |
| Oenothera oakesiana | Nachtkerze Küsten | Oakes's Evening Primrose | Onagraceae |
| Oenothera oehlkersii | Nachtkerze Oehlkers | Oehlkers's Evening Primrose | Onagraceae |
| Oenothera paradoxa | Nachtkerze Seltsame | Paradoxal Evening Primrose | Onagraceae |
| Oenothera parviflora | Nachtkerze Kleinblütige | Small- flowered Evening Primrose | Onagraceae |
| Oenothera pedemontana | Nachtkerze Piemonteser | Piedmont Evening Primrose | Onagraceae |
| Oenothera pellegrinii | Nachtkerze Pellegrinis | Pellegrini's Primrose | Onagraceae |
| Oenothera punctulata | Nachtkerze Feinpunktierte | Punctulate Primrose | Onagraceae |
| Oenothera pycnocarpa | Nachtkerze Kleinfrüchtige | Chicago Primrose | Onagraceae |
| Oenothera rosea | Nachtkerze Rosablütige | Rose Evening Primrose | Onagraceae |
| Oenothera royfraseri | Nachtkerze Roy Frasers | Roy Fraser's Evening Primrose | Onagraceae |
| Oenothera rubricalyx | Nachtkerze Blutrote | Blood Sepal Evening Primrose | Onagraceae |
| Oenothera rubricaulis | Nachtkerze Rotstängelige | Red- Stem Evening Primrose | Onagraceae |
| Oenothera rubricauloides | Nachtkerze Rote | Red Primrose | Onagraceae |
| Oenothera scandinavica | Nachtkerze Skandinavische | Scandinavian Evening Primrose | Onagraceae |
| Oenothera sesitensis | Nachtkerze Sesia | Sesia Evening Primrose | Onagraceae |
| Oenothera sinuosa | Prachtkerze Gebuchtete | Wavyleaf Beeblossom | Onagraceae |
| Oenothera speciosa | Nachtkerze Prächtige | Showy Evening Primrose | Onagraceae |
| Oenothera stucchii | Nachtkerze Stucchis | Stucchi's Evening Primrose | Onagraceae |
| Oenothera suaveolens | Nachtkerze Duftende | Smelling Evening Primrose | Onagraceae |
| Oenothera subterminalis | Nachtkerze Schlesische | Silesian Evening Primrose | Onagraceae |
| Oenothera stricta | Nachtkerze Steife | Chilean Evening Primrose | Onagraceae |
| Oenothera victorinii | Nachtkerze Victorins | Victorin's Evening Primrose | Onagraceae |
| Oenothera villosa | Nachtkerze Weißhaarige | Hairy Evening Primrose | Onagraceae |
| Oenothera wienii | Nachtkerze Danziger | Danzig Evening Primrose | Onagraceae |
| Oenothera x saxonica | Nachtkerze Sächsische | Saxonian Evening Primrose | Onagraceae |
| Olea euopaea | Ölbaum Europäischer | European Olive | Oleaceae |
| Omphalodes brassicifolia | Nabelnüsschen Kohlblättriges | Cabbage- leaved Navelwort | Boraginaceae |
| Omphalodes commutata | Nabelnüsschen Verwechseltes | Confused Navelwort | Boraginaceae |
| Omphalodes kuzinskyanae | Nabelnüsschen Kuzinsky's | Kuzinsky's Navelwort | Boraginaceae |
| Omphalodes linifolia | Nabelnüsschen Leinblättriges | Whiteflower Navelwort | Boraginaceae |
| Omphalodes littoralis | Nabelnüsschen Küsten | Coastal Navelwort | Boraginaceae |
| Omphalodes luciliae | Nabelnüsschen Lucilia | Lucilia's Navelwort | Boraginaceae |
| Omphalodes nitida | Nabelnüsschen Glänzendes | Shining Naverwort | Boraginaceae |
| Omphalodes runemarkii | Nabelnüsschen Runemarks | Runemarks Navelwort | Boraginaceae |
| Omphalodes scorpioides | Nabelnüsschen Wald | Wood Navelwort | Boraginaceae |
| Omphalodes verna | Nabelnüsschen Frühlings | Creeping Navelwort | Boraginaceae |
| Onobrychis aequidentata | Esparsette Gleichzähnige | Equal- toothed Sainfoin | Fabaceae |
| Onobrychis alba | Esparsette Ebenholzstrauchähnliche | Ebony- like Sainfoin | Fabaceae |
| Onobrychis aliacmona | Esparsette Aliakmon | Aliakmon Sainfoin | Fabaceae |
| Onobrychis arenaria | Esparsette Sand | Small Sainfoin | Fabaceae |
| Onobrychis argentea | Esparsette Silber | Silver Sainfoin | Fabaceae |
| Onobrychis caput- galli | Esparsette Hahnenkopf | Cock's- head Sainfoin | Fabaceae |
| Onobrychis citrina | Esparsette Zitronen | Lemon Sainfoin | Fabaceae |
| Onobrychis crista- galli | Esparsette Hahnenkamm | Cock's- comb Sainfoin | Fabaceae |
| Onobrychis degenii | Esparsette Degens | Degen's Sainfoin | Fabaceae |
| Onobrychis ebenoides | Esparsette Ebenholzstrauchähnliche | Ebony- like Sainfoin | Fabaceae |
| Onobrychis gracilis | Esparsette Zierliche | Slender Sainfoin | Fabaceae |
| Onobrychis humilis | Esparsette Niedrige | Dwarf Sainfoin | Fabaceae |
| Onobrychis matritensis | Esparsette Madrilenische | Madridian Sainfoin | Fabaceae |
| Onobrychis megalophylla | Esparsette Großblättrige | Macedonia Sainfoin | Fabaceae |
| Onobrychis montana | Esparsette Berg | Mountain Sainfoin | Fabaceae |
| Onobrychis peloponnesiaca | Esparsette Peloponnes | Peloponnes Sainfoin | Fabaceae |
| Onobrychis pindicola | Esparsette Pindos | Pindos Sainfoin | Fabaceae |
| Onobrychis pyrenaica | Esparsette Pyrenäen | Pyrenean Sainfoin | Fabaceae |
| Onobrychis reuteri | Esparsette Reuters | Reuter's Sainfoin | Fabaceae |
| Onobrychis saxatilis | Esparsette Felsen | Rock Sainfoin | Fabaceae |
| Onobrychis sphaciotica | Esparsette Sfakia | Sfakia Sainfoin | Fabaceae |
| Onobrychis stenorhiza | Esparsette Schmalwurzelige | Narrow- rooted Sainfoin | Fabaceae |
| Onobrychis supina | Esparsette Niederliegende | Supine Sainfoin | Fabaceae |
| Onobrychis viciifolia | Esparsette Berg | Mountain Sainfoin | Fabaceae |
| Onoclea sensibilis | Perlfarn Empfindlicher | Sensitive Fern | Onocleaceae |
| Ononis alba | Hauhechel Weiße | White Restharrow | Fabaceae |
| Ononis alopecuroides | Hauhechel Fuchsschwanz | Foxtail Restharrow | Fabaceae |
| Ononis aragonensis | Hauhechel Aragon | Aragon Restharrow | Fabaceae |
| Ononis arvensis | Hauhechel Bocks | Field Restharrow | Fabaceae |
| Ononis azcaratei | Hauhechel Azcarate | Azcarate Restharrow | Fabaceae |
| Ononis baetica | Hauhechel Baetische | Baetian Restharrow | Fabaceae |
| Ononis biflora | Hauhechel Zweiblütige | Two- flowered Restharrow | Fabaceae |
| Ononis broteriana | Hauhechel Broteros | Brotero's Restharrow | Fabaceae |
| Ononis cephalotes | Hauhechel Kopfige | Large flower- headed Restharrow | Fabaceae |
| Ononis cintrana | Hauhechel Sintra | Sintra Restharrow | Fabaceae |
| Ononis cossoniana | Hauhechel Cossons | Cosson's Restharrow | Fabaceae |
| Ononis crispa | Hauhechel Krause | Crispate Restharrow | Fabaceae |
| Ononis cristata | Hauhechel Mont Cenis | Mount Cenis Restharrow | Fabaceae |
| Ononis dentata | Hauhechel Gezähnte | Toothed restharrow | Fabaceae |
| Ononis diffusa | Hauhechel Ausgebreitete | Diffuse Restharrow | Fabaceae |
| Ononis filicaulis | Hauhechel Fadenstängelige | Thinstem Restharrow | Fabaceae |
| Ononis foetens | Hauhechel Stinkende | Austrian Restharrow | Fabaceae |
| Ononis fruticosa | Hauhechel Strauchige | Shrubby Restharrow | Fabaceae |
| Ononis hackelii | Hauhechel Hackels | Hackel's Restharrow | Fabaceae |
| Ononis hirta | Hauhechel Raue | Hairy Restharrow | Fabaceae |
| Ononis hispida | Hauhechel Borstige | Hispid Restharrow | Fabaceae |
| Ononis laxiflora | Hauhechel Lockerblütige | Lax- flowered Restharrow | Fabaceae |
| Ononis leucotricha | Hauhechel Weißhaariger | White- haired Restharrow | Fabaceae |
| Ononis masquillierii | Hauhechel Masquiller | Masquiller's Restharrow | Fabaceae |
| Ononis minutissima | Hauhechel Winzige | Tiny Restharrow | Fabaceae |
| Ononis mitissima | Hauhechel Mittelmeer | Mediterranean Restharrow | Fabaceae |
| Ononis natrix | Hauhechel Gelber | Yellow Restharrow | Fabaceae |
| Ononis oligophylla | Hauhechel Wenigblättrige | Few- leaved Restharrow | Fabaceae |
| Ononis ornithopodioides | Hauhechel Vogelfußähnliche | Bird Restharrow | Fabaceae |
| Ononis pendula | Hauhechel Hänge | Hanging Restharrow | Fabaceae |
| Ononis pinnata | Hauhechel Fiederblättrige | Pinnate Restharrow | Fabaceae |
| Ononis pubescens | Hauhechel Weichhaarige | Downy Restharrow | Fabaceae |
| Ononis pusilla | Hauhechel Zwerg | Dwarf Restharrow | Fabaceae |
| Ononis ramosissima | Hauhechel Vielästiger | Spanish Restharrow | Fabaceae |
| Ononis reclinata | Hauhechel Nickende | Small Restharrow | Fabaceae |
| Ononis repens | Hauhechel Kriechende | Common Restharrow | Fabaceae |
| Ononis reuteri | Hauhechel Reuter's | Reuter's Restharrow | Fabaceae |
| Ononis rotundifolia | Hauhechel Rundblättrige | Round- leaved Restharrow | Fabaceae |
| Ononis saxicola | Hauhechel Felsen | Rock Restharrow | Fabaceae |
| Ononis serrata | Hauhechel Gesägte | Serrate Restharrow | Fabaceae |
| Ononis sicula | Hauhechel Sizilianische | Sicilian Restharrow | Fabaceae |
| Ononis sieberi | Hauhechel Siebers | Sieber's Restharrow | Fabaceae |
| Ononis speciosa | Hauhechel Prächtige | Magnificent Restharrow | Fabaceae |
| Ononis spinosa | Hauhechel Dornige | Spiny Restharrow | Fabaceae |
| Ononis striata | Hauhechel Gestreifter | Striped Restharrow | Fabaceae |
| Ononis talaverae | Hauhechel Talaveras | Talavera's Restharrow | Fabaceae |
| Ononis tournefortii | Hauhechel Tourneforts | Tournefort's Restharrow | Fabaceae |
| Ononis tridentata | Hauhechel Dreizähnige | Three- teethed Restharrow | Fabaceae |
| Ononis varelae | Hauhechel Varelas | Varela's Restharrow | Fabaceae |
| Ononis variegata | Hauhechel Bunte | Villous Restharrow | Fabaceae |
| Ononis verae | Hauhechel Echte | Cretan Restharrow | Fabaceae |
| Ononis viscosa | Hauhechel Klebrige | Sticky Restharrow | Fabaceae |
| Onopordum acanthium | Eselsdistel Gewöhnliche | Scotch Cottonthistle | Asteraceae |
| Onopordum acaulon | Eselsdistel Stängellose | Stemless Cottonthistle | Asteraceae |
| Onopordum argolicum | Eselsdistel Argolische | Argolian Cottonthistle | Asteraceae |
| Onopordum boisserianum | Eselsdistel Boissiers | Boissier's Cottonthistle | Asteraceae |
| Onopordum bracteatum | Eselsdistel Verzweigte | Branched Cottonthistle | Asteraceae |
| Onopordum caulescens | Eselsdistel Stängel | Stalked Cottonthistle | Asteraceae |
| Onopordum corymbosum | Eselsdistel Doldenrispige | Corymbose Cottonthistle | Asteraceae |
| Onopordum eriocephalum | Eselsdistel Wollköpfige | Wooly Cottonthistle | Asteraceae |
| Onopordum illyricum | Eselsdistel Illyrische | Illyrian Cottonthistle | Asteraceae |
| Onopordum hinojense | Eselsdistel Hinojos- | Moor's Cottonthistle | Asteraceae |
| Onopordum horridum | Eselsdistel Schreckliche | Horrible Cottonthistle | Asteraceae |
| Onopordum laconicum | Eselsdistel Lakonische | Laconian Cottonthistle | Asteraceae |
| Onopordum macracanthum | Eselsdistel Langdornige | Long- thorned Cottonthistle | Asteraceae |
| Onopordum majorii | Eselsdistel Majors | Major's Cottonthistle | Asteraceae |
| Onopordum messeniacum | Eselsdistel Messenische | Messenian Cottonthistle | Asteraceae |
| Onopordum nervosum | Eselsdistel Starknervige | Moor's Cottonthistle | Asteraceae |
| Onopordum tauricum | Eselsdistel Taurische | Taurian Cottonthistle | Asteraceae |
| Onopordum x hortorum | Eselsdistel Garten | Garden Cottonthistle | Asteraceae |
| Onosma arenaria | Lotwurz Sand | Purple Goldendrop | Boraginaceae |
| Onosma aucheriana | Lotwurz Auchers | Aucher's Goldendrop | Boraginaceae |
| Onosma bubanii | Lotwurz Bubanis | Bubani's Goldendrop | Boraginaceae |
| Onosma erecta | Lotwurz Aufrechte | Upright Goldendrop | Boraginaceae |
| Onosma fastigiata | Lotwurz Ligurische | Ligurian Goldendrop | Boraginaceae |
| Onosma frutescens | Lotwurz Strauchige | Rocky Goldendrop | Boraginaceae |
| Onosma graeca | Lotwurz Griechische | Greek Goldendrop | Boraginaceae |
| Onosma heterophylla | Lotwurz Verschiedenblättrige | Various- leaved Goldendrop | Boraginaceae |
| Onosma kaheirei | Lotwurz Karl Heinz Rechinger | Karl- Heinz Rechingers Goldendrop | Boraginaceae |
| Onosma leptantha | Lotwurz Zartblütige | Delicate- flowered Goldendrop | Boraginaceae |
| Onosma pseudoarenaria | Lotwurz Zweifelhafte | Doubtful Goldendrop | Boraginaceae |
| Onosma sangiasense | Lotwurz Sangias | Mt. Sangias Goldendrop | Boraginaceae |
| Onosma spruneri | Lotwurz Spruner's | Spruners Goldendrop | Boraginaceae |
| Onosma tricerosperma | Lotwurz Dreihörnige | Threehorned Goldendrop | Boraginaceae |
| Onosma visianii | Lotwurz Dalmatinische | Visani's Goldendrop | Boraginaceae |
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