- order rhoeadales
- order rhynchocephalia
- order rickettsiales
- order rodentia
- order rosales
- order rubiales
- order salicales
- order salientia
- order santalales
- order sapindales
- order saprolegniales
- order sarcosporidia
- order sarraceniales
- order saurischia
- order sauropterygia
- order scandentia
- order sclerodermatales
- order scleroparei
- order scorpionida
- order scrophulariales
- order secotiales
- order selaginellales
- order siluriformes
- order siphonaptera
- order siphonophora
- order sirenia
- order solenichthyes
- order solenogastres
- order spatangoida
- order sphaeriales
- order sphaerocarpales
- order sphagnales
- order sphenisciformes
- order spirochaetales
- order squamata
- order stegocephalia
- order stereospondyli
- order stomatopoda
- order strigiformes
- order struthioniformes
- order synentognathi
- order taxales
- order temnospondyli
- order testacea
- order testudinata
- order testudines
- order tetraodontiformes
- order thecodontia
- order therapsida
- order thymelaeales
- order thysanoptera
- order thysanura
- order tinamiformes
- order torpediniformes
- order tremellales
- order trichoptera
- order trogoniformes
- order tuberales
- order tubulidentata
- order tulostomatales
- order ulvales
- order umbellales
- order uredinales
- order urodella
- order uropygi
- order urticales
- order ustilaginales
- order volvocales
- order xiphosura
- order xyridales
- order zeomorphi
- order zygnemales
- order zygnematales
- order-chenopodiales
- ordered
- ordered series
- orderer
- ordering
- orderliness
- orderly
- orderly sergeant
- ordinal
- ordinal number
- ordinance
- ordinand
- ordinarily
- ordinariness
- ordinary
- ordinary annuity
- ordinary bicycle
- ordinary care
- ordinary life insurance
- ordinary shares
- ordinate
- ordination
- ordnance
- ordnance store
- ordnance survey
- ordovician
- ordovician period
- ordure
- ore
- ore bed
- ore dressing
- ore processing
- oread
- oreamnos
- oreamnos americanus
- orectolobidae
- orectolobus
- orectolobus barbatus
- oregano
- oregon
- oregon alder
- oregon ash
- oregon cedar
- oregon crab apple
- oregon fir
- oregon grape
- oregon holly grape
- oregon jargon
- oregon larch
- oregon lily
- oregon maple
- oregon myrtle
- oregon oak
- oregon pine
- oregon white oak
- oregonian
- oreide
- oreo
- oreo cookie
- oreopteris
- oreopteris limbosperma
- oreortyx
- oreortyx picta palmeri
- orestes
- orff
- organ
- organ donor
- organ loft
- organ of corti
- organ of hearing
- organ of speech
- organ pipe
- organ stop
- organ transplant
- organdie
- organdy
- organelle
- organ-grinder
- organic
- organic brain syndrome
- organic chemistry
- organic compound
- organic disorder
- organic evolution
- organic fertiliser
- organic fertilizer
- organic law
- organic light-emitting diode
- organic phenomenon
- organic process
- organic structure
- organically
- organicism
- organicistic
- organification
- organisation
- organisational
- organise
- organised
- organiser
- organism
- organismal
- organismic
- organist
- organization
- organization chart
- organization expense
- organization for the prohibition of chemical weapons
- organization man
- organization of american states
- organization of petroleum-exporting countries
- organization of the oppressed on earth
- organizational
- organizationally
- organize
- organized
- organized crime
- organized labor
- organized religion
- organizer
- organon
- organophosphate
- organophosphate nerve agent
- organs
- organza
- orgasm
- orgiastic