| Symbol | Scientific name | Common name/Accepted name (for synonyms) | Family |
| ORPE | Orbexilum pedunculatum (P. Mill.) Rydb. | Sampson's snakeroot | Fabaceae
|
| ORSI | Orbexilum simplex (Nutt. ex Torr. & Gray) Rydb. | singlestem leather-root | Fabaceae
|
| ORST | Orbexilum stipulatum (Torr. & Gray) Rydb. | largestipule leather-root | Fabaceae
|
| ORVI | Orbexilum virgatum (Nutt.) Rydb. | pineland leather-root | Fabaceae
|
| ORBEX | Orbexilum Raf. | leather-root | Fabaceae
|
| ORBA5 | Orbignya barbosiana Burret | babassu | Arecaceae
|
| ORCO9 | Orbignya cohune (C. Martius) Dahlgren ex Standley | Cohune palm | Arecaceae
|
| ORSP4 | Orbignya speciosa (C. Martius) Barb. Rodr. | Orbignya barbosiana | Arecaceae
|
| ORBIG | Orbignya C. Martius ex Endl. | orbignya | Arecaceae
|
| ORAR | Orchis aristata Fisch. ex Lindl. | Dactylorhiza aristata var. aristata | Orchidaceae
|
| ORMO5 | Orchis morio L. | green-winged orchid | Orchidaceae
|
| ORROL | Orchis rotundifolia Banks ex Pursh var. lineata Mousley | Amerorchis rotundifolia | Orchidaceae
|
| ORRO3 | Orchis rotundifolia Banks ex Pursh | Amerorchis rotundifolia | Orchidaceae
|
| ORSP2 | Orchis spectabilis L. | Galearis spectabilis | Orchidaceae
|
| ORCHI2 | Orchis L. | orchis | Orchidaceae
|
| ORCAI | Orcuttia californica Vasey var. inaequalis (Hoover) Hoover | Orcuttia inaequalis | Poaceae
|
| ORCAV | Orcuttia californica Vasey var. viscida Hoover | Orcuttia viscida | Poaceae
|
| ORCA | Orcuttia californica Vasey | California Orcutt grass | Poaceae
|
| ORGR | Orcuttia greenei Vasey | Tuctoria greenei | Poaceae
|
| ORIN | Orcuttia inaequalis Hoover | San Joaquin Valley Orcutt grass | Poaceae
|
| ORMU2 | Orcuttia mucronata Crampton | Tuctoria mucronata | Poaceae
|
| ORPI | Orcuttia pilosa Hoover | hairy Orcutt grass | Poaceae
|
| ORTE | Orcuttia tenuis A.S. Hitchc. | slender Orcutt grass | Poaceae
|
| ORVI2 | Orcuttia viscida (Hoover) J. Reeder | Sacramento Orcutt grass | Poaceae
|
| ORCUT | Orcuttia Vasey | Orcutt grass | Poaceae
|
| ORMA7 | Oreas martiana (Hoppe & Hornsch. in Hornsch.) Brid. | oreas moss | Dicranaceae
|
| OREAS | Oreas Brid. | oreas moss | Dicranaceae
|
| OREL | Oreastrum elatum Greene | Oreostemma elatum | Asteraceae
|
| ORDEN2 | Oreobatus deliciosus (James ex Torr.) Rydb. ssp. neomexicanus (Gray) W.A. Weber | Rubus neomexicanus | Rosaceae
|
| ORDE2 | Oreobatus deliciosus (James ex Torr.) Rydb. | Rubus deliciosus | Rosaceae
|
| ORFU2 | Oreobolus furcatus Mann | Hawai'i Island sedge | Cyperaceae
|
| OREOB | Oreobolus R. Br. | oreobolus | Cyperaceae
|
| ORHE2 | Oreobroma heckneri Morton | Lewisia cotyledon var. heckneri | Portulacaceae
|
| ORLO2 | Oreobroma longipetalum Piper | Lewisia longipetala | Portulacaceae
|
| ORNE2 | Oreobroma nevadense (Gray) T.J. Howell | Lewisia nevadensis | Portulacaceae
|
| ORPY2 | Oreobroma pygmaeum (Gray) T.J. Howell | Lewisia pygmaea | Portulacaceae
|
| ORAB | Oreocarya abortiva Greene | Cryptantha cinerea var. abortiva | Boraginaceae
|
| ORAP | Oreocarya aperta Eastw. | Cryptantha aperta | Boraginaceae
|
| ORBA4 | Oreocarya bakeri Greene | Cryptantha bakeri | Boraginaceae
|
| ORBR3 | Oreocarya breviflora Osterhout | Cryptantha breviflora | Boraginaceae
|
| ORCA8 | Oreocarya caespitosa A. Nels. | Cryptantha caespitosa | Boraginaceae
|
| ORCA11 | Oreocarya cana A. Nels. | Cryptantha cana | Boraginaceae
|
| ORCE2 | Oreocarya celosioides Eastw. | Cryptantha celosioides | Boraginaceae
|
| ORCO6 | Oreocarya confertiflora Greene | Cryptantha confertiflora | Boraginaceae
|
| ORCR2 | Oreocarya crymophila (I.M. Johnston) Jepson & Hoover | Cryptantha crymophila | Boraginaceae
|
| OREL2 | Oreocarya elata Eastw. | Cryptantha elata | Boraginaceae
|
| ORFL2 | Oreocarya flava A. Nels. | Cryptantha flava | Boraginaceae
|
| ORFL3 | Oreocarya flavoculata A. Nels. | Cryptantha flavoculata | Boraginaceae
|
| ORFU3 | Oreocarya fulvocanescens (S. Wats.) Greene | Cryptantha fulvocanescens var. fulvocanescens | Boraginaceae
|
| ORGL | Oreocarya glomerata (Pursh) Greene, non Lehm. ex Fisch. | Cryptantha celosioides | Boraginaceae
|
| ORHO | Oreocarya hoffmannii (I.M. Johnston) Abrams | Cryptantha hoffmannii | Boraginaceae
|
| ORHUN | Oreocarya humilis (Gray) Greene ssp. nana (Eastw.) W.A. Weber | Cryptantha humilis | Boraginaceae
|
| ORHU2 | Oreocarya humilis (Gray) Greene | Cryptantha humilis | Boraginaceae
|
| ORLE | Oreocarya leucophaea (Dougl. ex Lehm.) Greene | Cryptantha leucophaea | Boraginaceae
|
| ORLO | Oreocarya longiflora A. Nels. | Cryptantha longiflora | Boraginaceae
|
| ORMA5 | Oreocarya macounii Eastw. | Cryptantha celosioides | Boraginaceae
|
| ORME | Oreocarya mensana M.E. Jones | Cryptantha mensana | Boraginaceae
|
| ORNI | Oreocarya nitida Greene | Cryptantha fulvocanescens var. nitida | Boraginaceae
|
| ORNU2 | Oreocarya nubigena Greene | Cryptantha nubigena | Boraginaceae
|
| OROS | Oreocarya osterhoutii Payson | Cryptantha osterhoutii | Boraginaceae
|
| ORPA4 | Oreocarya paradoxa A. Nels. | Cryptantha paradoxa | Boraginaceae
|
| ORPR | Oreocarya propria A. Nels. & J.F. Macbr. | Cryptantha propria | Boraginaceae
|
| ORPU14 | Oreocarya pustulosa Rydb. | Cryptantha cinerea var. pustulosa | Boraginaceae
|
| ORRO4 | Oreocarya rollinsii (I.M. Johnston) W.A. Weber | Cryptantha rollinsii | Boraginaceae
|
| ORSE3 | Oreocarya sericea (Gray) Greene | Cryptantha sericea | Boraginaceae
|
| ORSH | Oreocarya sheldonii Brand | Cryptantha celosioides | Boraginaceae
|
| ORSP3 | Oreocarya spiculifera Piper | Cryptantha spiculifera | Boraginaceae
|
| ORST2 | Oreocarya stricta Osterhout | Cryptantha stricta | Boraginaceae
|
| ORSU3 | Oreocarya subretusa (I.M. Johnston) Abrams | Cryptantha sobolifera | Boraginaceae
|
| ORSU2 | Oreocarya suffruticosa (Piper) Greene | Cryptantha cinerea var. jamesii | Boraginaceae
|
| ORTH2 | Oreocarya thompsonii (I.M. Johnston) Abrams | Cryptantha thompsonii | Boraginaceae
|
| ORTH3 | Oreocarya thyrsiflora Greene | Cryptantha thyrsiflora | Boraginaceae
|
| ORTU | Oreocarya tumulosa Payson | Cryptantha tumulosa | Boraginaceae
|
| ORVI4 | Oreocarya virgata (Porter) Greene | Cryptantha virgata | Boraginaceae
|
| ORVI3 | Oreocarya virginensis (M.E. Jones) J.F. Macbr. | Cryptantha virginensis | Boraginaceae
|
| ORWE2 | Oreocarya weberi (I.M. Johnston) W.A. Weber | Cryptantha weberi | Boraginaceae
|
| ORPA3 | Oreochrysum parryi (Gray) Rydb. | Parry's goldenrod | Asteraceae
|
| OREOC | Oreochrysum Rydb. | goldenrod | Asteraceae
|
| ORAR2 | Oreolirion arizonicum (Rothrock) Bickn. | Sisyrinchium arizonicum | Iridaceae
|
| ORCL | Oreonana clementis (M.E. Jones) Jepson | pygmy mountainparsley | Apiaceae
|
| ORPU5 | Oreonana purpurascens Shevock & Constance | purple mountainparsley | Apiaceae
|
| ORVE | Oreonana vestita (S. Wats.) Jepson | woolly mountainparsley | Apiaceae
|
| OREON | Oreonana Jepson | mountainparsley | Apiaceae
|
| ORLI4 | Oreopteris limbosperma auct. non (All.) Holub | Thelypteris quelpaertensis | Thelypteridaceae
|
| ORQU | Oreopteris quelpaertensis (Christ) Holub | Thelypteris quelpaertensis | Thelypteridaceae
|
| ORAL2 | Oreosedum album (L.) Grulich | Sedum album | Crassulaceae
|
| ORALA2 | Oreostemma alpigenum (Torr. & Gray) Greene var. alpigenum | tundra aster | Asteraceae
|
| ORALA3 | Oreostemma alpigenum (Torr. & Gray) Greene var. andersonii (Gray) Nesom | tundra aster | Asteraceae
|
| ORALH | Oreostemma alpigenum (Torr. & Gray) Greene var. haydenii (Porter) Nesom | tundra aster | Asteraceae
|
| ORAL4 | Oreostemma alpigenum (Torr. & Gray) Greene | tundra aster | Asteraceae
|
| OREL4 | Oreostemma elatum (Greene) Greene | Plumas alpine aster | Asteraceae
|
| ORPE4 | Oreostemma peirsonii (C.W. Sharsmith) Nesom | Peirson's aster | Asteraceae
|
| OREOS | Oreostemma Greene | aster | Asteraceae
|
| ORALA | Oreoxis alpina (Gray) Coult. & Rose ssp. alpina | alpine oreoxis | Apiaceae
|
| ORALP | Oreoxis alpina (Gray) Coult. & Rose ssp. puberulenta W.A. Weber | alpine oreoxis | Apiaceae
|
| ORAL | Oreoxis alpina (Gray) Coult. & Rose | alpine oreoxis | Apiaceae
|
| ORBA | Oreoxis bakeri Coult. & Rose | Baker's alpineparsley | Apiaceae
|
| ORHU | Oreoxis humilis Raf. | Rocky Mountain alpineparsley | Apiaceae
|
| ORMA4 | Oreoxis macdougalii (Coult. & Rose) Rydb. | Aletes macdougalii ssp. macdougalii | Apiaceae
|
| ORTR | Oreoxis trotteri Welsh & Goodrich | Trotter's alpineparsley | Apiaceae
|
| OREOX | Oreoxis Raf. | oreoxis | Apiaceae
|