| Symbol | Scientific name | Common name/Accepted name (for synonyms) | Family |
| OROBA | Orobanche L. | broomrape | Orobanchaceae
|
| ORMY | Orobus myrtifolius (Muhl. ex Willd.) A. Hall | Lathyrus palustris | Fabaceae
|
| ORTH | Orochaenactis thysanocarpha (Gray) Coville | California mountainpincushion | Asteraceae
|
| OROCH | Orochaenactis Coville | orochaenactis | Asteraceae
|
| ORFU | Orogenia fusiformis S. Wats. | California Indian potato | Apiaceae
|
| ORLIL2 | Orogenia linearifolia S. Wats. var. lata Payson | Orogenia linearifolia | Apiaceae
|
| ORLI | Orogenia linearifolia S. Wats. | Great Basin Indian potato | Apiaceae
|
| OROGE | Orogenia S. Wats. | Indian potato | Apiaceae
|
| ORAQ | Orontium aquaticum L. | goldenclub | Araceae
|
| ORONT | Orontium L. | goldenclub | Araceae
|
| ORAR4 | Orophaca aretioides (M.E. Jones) Rydb. | Astragalus aretioides | Fabaceae
|
| ORBA3 | Orophaca barrii (Barneby ) Isely | Astragalus barrii | Fabaceae
|
| ORCA7 | Orophaca caespitosa (Nutt.) Britt. | Astragalus spatulatus | Fabaceae
|
| ORHY2 | Orophaca hyalina (M.E. Jones) Isely | Astragalus hyalinus | Fabaceae
|
| ORPR2 | Orophaca proimantha (Barneby) Isely | Astragalus proimanthus | Fabaceae
|
| ORSE2 | Orophaca sericea auct. non (Nutt.) Britt. | Astragalus sericoleucus | Fabaceae
|
| ORTR2 | Orophaca tridactylica (Gray) Rydb. | Astragalus tridactylicus | Fabaceae
|
| ORTR3 | Orophaca triphylla Britt. | Astragalus gilviflorus var. gilviflorus | Fabaceae
|
| ORAT2 | Orphniospora atrata (Sm.) Poelt | Orphniospora moriopsis | Fuscideaceae
|
| ORMO2 | Orphniospora moriopsis (A. Massal.) D. Hawksw. | | Fuscideaceae
|
| ORPHN | Orphniospora Korber | orphniospora | Fuscideaceae
|
| ORSEO | Orthilia secunda (L.) House ssp. obtusata (Turcz.) B�cher | Orthilia secunda | Pyrolaceae
|
| ORSEO2 | Orthilia secunda (L.) House var. obtusata (Turcz.) House | Orthilia secunda | Pyrolaceae
|
| ORSE | Orthilia secunda (L.) House | sidebells wintergreen | Pyrolaceae
|
| ORTHI | Orthilia Raf. | orthilia | Pyrolaceae
|
| ORDO | Orthiopteris domingensis (Spreng.) Copeland | Saccoloma domingense | Dennstaedtiaceae
|
| ORIN2 | Orthiopteris inaequalis (Kunze) Copeland | Saccoloma inaequale | Dennstaedtiaceae
|
| ORAT | Orthocarpus attenuatus Gray | Castilleja attenuata | Scrophulariaceae
|
| ORBA2 | Orthocarpus barbatus Cotton | Grand Coulee owl's-clover | Scrophulariaceae
|
| ORBRA | Orthocarpus bracteosus Benth. var. albus Keck | Orthocarpus bracteosus | Scrophulariaceae
|
| ORBR | Orthocarpus bracteosus Benth. | rosy owl's-clover | Scrophulariaceae
|
| ORBR2 | Orthocarpus brevistylus Hoover | Castilleja brevistyla | Scrophulariaceae
|
| ORCAS | Orthocarpus campestris Benth. var. succulentus Hoover | Castilleja campestris ssp. succulenta | Scrophulariaceae
|
| ORCA3 | Orthocarpus campestris Benth. | Castilleja campestris ssp. campestris | Scrophulariaceae
|
| ORCAH | Orthocarpus castillejoides Benth. var. humboldtiensis Keck | Castilleja ambigua ssp. humboldtiensis | Scrophulariaceae
|
| ORCAI2 | Orthocarpus castillejoides Benth. var. insalutatus Jepson | Castilleja ambigua ssp. insalutata | Scrophulariaceae
|
| ORCA4 | Orthocarpus castillejoides Benth. | Castilleja ambigua ssp. ambigua | Scrophulariaceae
|
| ORCOC | Orthocarpus copelandii Eastw. var. cryptanthus (Piper) Keck | Orthocarpus cuspidatus ssp. cryptanthus | Scrophulariaceae
|
| ORCO | Orthocarpus copelandii Eastw. | Orthocarpus cuspidatus ssp. copelandii | Scrophulariaceae
|
| ORCUC | Orthocarpus cuspidatus Greene ssp. copelandii (Eastw.) Chuang & Heckard | Copeland's owl's-clover | Scrophulariaceae
|
| ORCUC2 | Orthocarpus cuspidatus Greene ssp. cryptanthus (Piper) Chuang & Heckard | cryptantha owl's-clover | Scrophulariaceae
|
| ORCUC3 | Orthocarpus cuspidatus Greene ssp. cuspidatus | Siskiyou Mountain owl's-clover | Scrophulariaceae
|
| ORCU | Orthocarpus cuspidatus Greene | toothed owl's-clover | Scrophulariaceae
|
| ORDEG | Orthocarpus densiflorus Benth. var. gracilis (Benth.) Keck | Castilleja densiflora ssp. gracilis | Scrophulariaceae
|
| ORDEN | Orthocarpus densiflorus Benth. var. noctuinus (Eastw.) J.T. Howell | Castilleja densiflora ssp. densiflora | Scrophulariaceae
|
| ORDEO | Orthocarpus densiflorus Benth. var. obispoensis Keck | Castilleja densiflora ssp. obispoensis | Scrophulariaceae
|
| ORDE | Orthocarpus densiflorus Benth. | Castilleja densiflora ssp. densiflora | Scrophulariaceae
|
| ORERG | Orthocarpus erianthus Benth. var. gratiosus Jepson & Tracy | Triphysaria eriantha ssp. rosea | Scrophulariaceae
|
| ORERM | Orthocarpus erianthus Benth. var. micranthus (Greene ex Heller) Jepson | Triphysaria micrantha | Scrophulariaceae
|
| ORERR | Orthocarpus erianthus Benth. var. roseus (Gray) Greene | Triphysaria eriantha ssp. rosea | Scrophulariaceae
|
| ORER | Orthocarpus erianthus Benth. | Triphysaria eriantha ssp. eriantha | Scrophulariaceae
|
| OREX3 | Orthocarpus exsertus Heller | Castilleja exserta ssp. exserta | Scrophulariaceae
|
| ORFAA | Orthocarpus faucibarbatus Gray ssp. albidus Keck | Triphysaria versicolor ssp. versicolor | Scrophulariaceae
|
| ORFAA2 | Orthocarpus faucibarbatus Gray var. albidus (Keck) J.T. Howell | Triphysaria versicolor ssp. versicolor | Scrophulariaceae
|
| ORFA2 | Orthocarpus faucibarbatus Gray | Triphysaria versicolor ssp. faucibarbatus | Scrophulariaceae
|
| ORFL | Orthocarpus floribundus Benth. | Triphysaria floribunda | Scrophulariaceae
|
| ORHI | Orthocarpus hispidus Benth. | Castilleja tenuis | Scrophulariaceae
|
| ORIM | Orthocarpus imbricatus Torr. ex S. Wats. | mountain owl's-clover | Scrophulariaceae
|
| ORLA | Orthocarpus lacerus Benth. | Castilleja lacera | Scrophulariaceae
|
| ORLA2 | Orthocarpus lasiorhynchus Gray | Castilleja lasiorhyncha | Scrophulariaceae
|
| ORLI5 | Orthocarpus lineariilobus Benth. | Castilleja lineariiloba | Scrophulariaceae
|
| ORLIB | Orthocarpus lithospermoides Benth. var. bicolor (Heller) Jepson | Castilleja rubicundula ssp. lithospermoides | Scrophulariaceae
|
| ORLI3 | Orthocarpus lithospermoides Benth. | Castilleja rubicundula ssp. lithospermoides | Scrophulariaceae
|
| ORLU2 | Orthocarpus luteus Nutt. | yellow owl's-clover | Scrophulariaceae
|
| ORMI5 | Orthocarpus micranthus Greene ex Heller | Triphysaria micrantha | Scrophulariaceae
|
| ORPA | Orthocarpus pachystachyus Gray | Shasta owl's-clover | Scrophulariaceae
|
| ORPI4 | Orthocarpus pilosus S. Wats. | Castilleja pilosa var. pilosa | Scrophulariaceae
|
| ORPUL | Orthocarpus purpurascens Benth. var. latifolius S. Wats. | Castilleja exserta ssp. latifolia | Scrophulariaceae
|
| ORPUO | Orthocarpus purpurascens Benth. var. ornatus Jepson | Castilleja exserta ssp. venusta | Scrophulariaceae
|
| ORPUP | Orthocarpus purpurascens Benth. var. pallidus Keck | Castilleja exserta ssp. exserta | Scrophulariaceae
|
| ORPUP3 | Orthocarpus purpurascens Benth. var. palmeri Gray | Castilleja exserta ssp. exserta | Scrophulariaceae
|
| ORPUV | Orthocarpus purpurascens Benth. var. venustus (Heller) Keck | Castilleja exserta ssp. venusta | Scrophulariaceae
|
| ORPU | Orthocarpus purpurascens Benth. | Castilleja exserta ssp. exserta | Scrophulariaceae
|
| ORPU2 | Orthocarpus purpureoalbus Gray ex S. Wats. | purplewhite owl's-clover | Scrophulariaceae
|
| ORPUD | Orthocarpus pusillus Benth. var. densiuscuus (Gandog.) Keck | Triphysaria pusilla | Scrophulariaceae
|
| ORPU3 | Orthocarpus pusillus Benth. | Triphysaria pusilla | Scrophulariaceae
|
| ORSU | Orthocarpus succulentus (Hoover) Hoover | Castilleja campestris ssp. succulenta | Scrophulariaceae
|
| ORTE2 | Orthocarpus tenuifolius (Pursh) Benth. | thinleaved owl's-clover | Scrophulariaceae
|
| ORTOH | Orthocarpus tolmiei Hook. & Arn. ssp. holmgreniorum Chuang & Heckard | Noel's owl's-clover | Scrophulariaceae
|
| ORTOT | Orthocarpus tolmiei Hook. & Arn. ssp. tolmiei | Tolmie's owl's-clover | Scrophulariaceae
|
| ORTO | Orthocarpus tolmiei Hook. & Arn. | Tolmie's owl's-clover | Scrophulariaceae
|
| ORTHO | Orthocarpus Nutt. | owl's-clover | Scrophulariaceae
|
| ORFL5 | Orthodicranum flagellare (Hedw.) Loeske | Dicranum flagellare | Dicranaceae
|
| ORMO3 | Orthodicranum montanum (Hedw.) Loeske | Dicranum montanum | Dicranaceae
|
| ORRH | Orthodicranum rhabdocarpum (Sull.) Holz. | Dicranum rhabdocarpum | Dicranaceae
|
| ORDI4 | Orthodon dianthera (Buch.-Ham. ex Roxb.) Hand.-Maz. | Mosla dianthera | Lamiaceae
|
| ORGR5 | Orthodontium gracile (Wils. in Sm. & Sowerby) Schwaegr. ex B.S.G. | orthodontium moss | Bryaceae
|
| ORPE2 | Orthodontium pellucens (Hook.) Bruch & Schimp. in B.S.G. | orthodontium moss | Bryaceae
|
| ORTHO5 | Orthodontium Schwaegr. | orthodontium moss | Bryaceae
|
| ORTHO7 | Orthosiphon Benth. | orthosiphon | Lamiaceae
|
| ORAC | Orthothecium acuminatum Bryhn | acuminate orthothecium moss | Hypnaceae
|
| ORCHC | Orthothecium chryseum (Schwaegr. in Schultes) Schimp. in B.S.G. var. chryseum | orthothecium moss | Hypnaceae
|
| ORCHC2 | Orthothecium chryseum (Schwaegr. in Schultes) Schimp. in B.S.G. var. cochlearifolium (Lindb.) Limpr. | orthothecium moss | Hypnaceae
|
| ORCHL | Orthothecium chryseum (Schwaegr. in Schultes) Schimp. in B.S.G. var. lapponicum (Schimp.) Lindb. in G. Roth | Orthothecium chryseum var. chryseum | Hypnaceae
|
| ORCH2 | Orthothecium chryseum (Schwaegr. in Schultes) Schimp. in B.S.G. | orthothecium moss | Hypnaceae
|
| ORDI2 | Orthothecium diminutivum (Grout) Crum et al. | diminutive orthothecium moss | Hypnaceae
|
| ORIN70 | Orthothecium intricatum (Hartm.) Schimp. in B.S.G. | intricate orthothecium moss | Hypnaceae
|
| ORLA5 | Orthothecium lapponicum (Schimp.) Hartm. | Orthothecium chryseum var. chryseum | Hypnaceae
|
| ORRU4 | Orthothecium rubellum (Mitt.) Kindb. | Orthothecium strictum | Hypnaceae
|
| ORRU2 | Orthothecium rufescens (Brid.) Schimp. in B.S.G. | orthothecium moss | Hypnaceae
|
| ORST70 | Orthothecium strictum Lor. | orthothecium moss | Hypnaceae
|