| obelós |  | spit, nail | obel-[127] | ὀβελός (obelós), ὀβελίσκος (obelískos) | metobelus, obelisk, obelism, obelus |
| obolós | | | obol-[128] | ὀβολός, ὀβολοῦ (obolós, oboloû) | obol, obolus |
| ōkeanós | | | ocean-[129] | ὠκεανός (ōkeanós) | Oceania, oceanic |
| ókhlos |  | crowd, mob | ochl-[130] | ὄχλος, ὄχλου (ókhlos, ókhlou) | enochlophobia, ochlocracy, ochlophobia |
| oktṓ |  | eight | oct-[131] | ὀκτώ (oktṓ), ὀκτάς (oktás), ὀκτάκις (oktákis) “eight times” | hemi-octahedron, octad, octadic, octagon, octahedron, octameter, octode |
| hodós |  | path, way | od- | ὁδός (hodós) | anode, diode, odometer, parodos, pentode, tetrode, triode |
| odoús |  | tooth | odont-[132] | ὀδούς, ὀδόντος (odoús, odóntos) | anodontia, conodont, cynodont, dicynodont, dysodontiasis, macrodontia, mastodon, odontoid, odontology, odontophore, orthodontics, orthodontist, pedodontics, periodontal, smilodon, Thrinaxodon, Zanclodon |
| odúnē |  | pain | odyn-[133] | ὀδύνη (odúnē) | allodynia, anodyne, glossodynia, mastodynia, pleurodynia |
| woîkos |  | house | oec-[134] | ϝοῖκος (woîkos), οἶκος (oîkos) | andromonecy, archdiocese, autoecious, autoecism, dioecious, dioecy, ecesis, ecologist, ecology, economics, economist, economize, economy, ecoregion, ecosystem, ecumenic, ecumenism, gynomonoecy, heteroecious, heteroecism, heteroecy, homoecious, monoecious, monoecy, oecology, oeconomus, oecumenic, oikology, oikophobia, palaeoecology, paleoecology, parish, paroecious, perioeci, trioecious |
| oîdos |  | swell, swollen | oed-, ed- | οἶδος (oîdos), οἰδεῖν (oideîn), οἴδημα, οἴδηματος (oídēma, oídēmatos) | angioedema, edema, edematous, oedema, oedematous |
| woînos |  | wine | oen-[135] | ϝοῖνος (woînos), οἶνος (oînos) | enology, oenochoe, oenologist, oenology, oenophile, oenophilia, oinochoe |
| oisophágos |  | gullet | oesophag-[136] | οἰσοφάγος (oisophágos) | oesophagectomy, oesophagitis, oesophagus |
| oîstros |  | gadfly, sting | oestr-[137] | οἶστρος (oîstros), οἰστράω, οἴστρησις, οἴστρημα | anestrous, anestrus, anoestrus, estrogen, estrogenic, estrus, oestrone, oestrus |
| ógdoos |  | eighth | ogdo-[138] | ὄγδοος (ógdoos), ὀγδοάς, ὀγδοάδος (ogdoás, ogdoádos) | ogdoad |
| oeidēs |  | like | -oid | -οειδής (-oeidēs) | asteroid, mucoid, organoid |
| olígos |  | few | olig-[140] | ὀλίγος (olígos) | oligarchy, Oligocene, oligopoly, oligosaccharide, oligotrophic |
| ōmós |  | raw | om-[141] | ὠμός (ōmós), ὠμότης (ōmótēs) “rawness” | omophagia, Omophagus |
| ômos |  | shoulder | om-[142] | ὦμος (ômos), ὠμία (ōmía) | acromion, omohyoid, omophorion |
| |  | morbid growth, tumor | -oma | -ωμα | melanoma |
| ómbros |  | rain | ombr-[143] | ὄμβρος (ómbros) | ombrogenous, ombrology, ombrometer, Ombrophila, ombrophilous, ombrophobe, ombrotrophic |
| ómma |  | eye | ommat- | ὁράω, ὦμμαι, ὄμμα, ὄμματος (ómma, ómmatos) | ommatidium, ommatophore |
| omphalós |  | navel | omphal-[144] | ὀμφαλός (omphalós) | omphalectomy, omphalic, omphalopagus, omphalophobia, paromphalocele |
| ónos |  | ass | on-[145] | ὄνος (ónos), ὀνίσκος (onískos) | Oniscidea, Oniscomorpha |
| ónkinos |  | barb, hook | onc-[146] | ὄγκος, ὄγκινος (ónkinos) | Oncinocalyx, Oncorhynchus |
| ónkos |  | bulk | onc-[147] (ΕΓΚ[148]) | ἐνεγκεῖν, ὄγκος (ónkos), ὀγκόω, ὀγκωτός, ὄγκωσις, ὄγκωμα | oncocyte, oncocytoma, oncogenesis, oncologist, oncology |
| óneiros |  | dream | oneir-[149] | ὄνειρος (óneiros), ὀνειρώσσω, ὀνείρωξις (oneírōxis) | oneiric, oneirism, oneirocritic, Oneirocritica, Oneirodidae, oneirogen, oneirogenic, Oneiroi, oneiroid, oneirology, oneiromancy, oneironaut, oneironautics, oneirophobia, oneirophrenia, oneiroscopy |
| ônos |  | price | oni-[150] | ὦνος (ônos), ὤνιος (ṓnios) | oniochalasia, oniomania, oniomaniac |
| ónoma |  | name | onomat-[151] | ὄνομα, ὀνόματος (ónoma, onómatos), ὀνομάζω, ὀνομαστικός (onomastikós) | antonomasia, onomasiology, onomastic, onomasticon, onomastics, onomasty, onomatology, onomatophore, onomatopoeia |
| óntos |  | being, existence | ont- | ὄντος, ὀντότης (óntos, ontótēs), οὐσία (ousía) | dysontogenesis, homoiousia, homoousia, monoousious, ontogenesis, ontogenetic, ontogeny, ontology, ousia, parousia |
| ónux |  | claw | onych-[152] | ὄνυξ, ὄνυχος (ónux, ónukhos), ὀνύχιον (onúkhion), ὀνύχινος, ὀνυχίζω | hapalonychia, Mesonychia, mesonychid, onychectomy, onycholysis, onychomancy, onychomycosis, onychophagia, onychophagy, onychorrhexis, onyx, paronychia, sardonyx |
| ónuma | | name | onym-[153] | ὄνυμα (ónuma) | acronym, allonym, anonymous, antonym, autonym, caconym, cryptonym, eponym, eponymous, eponymy, euonym, homonym, hyperonym, hyponym, hyponymy, meronym, meronymy, metonym, metonymy, metronymic, paronym, paronymous, pseudonym, pseudonymous, synonym, synonymous, synonymy, tautonym, tautonymous, tautonymy, troponym, troponymy, xenonym, xenonymy |
| ōión |  | egg | oo-[154] | ᾠόν (ōión) | bottarga, dioon, epoophoron, oidioid, oidium, ooblast, oocyst, oocyte, oocytogenesis, oogamete, oogamous, oogamy, oogenesis, oogonium, ooid, oolite, oolith, oology, oomancy, oophagy, oophorectomy, oophoron, ootheca, ootid, ootidogenesis, paroophoron |
| opḗ |  | hole | op-[155] | ὀπή (opḗ), ὀπαῖον (opaîon) | metope, opaion |
| óphis |  | snake | ophi-[156] | ὄφις, ὄφεως (óphis, ópheōs) | Brachyurophis, ophicephalous, Ophicephalus, Ophiceras, Ophiclinus, ophidiophobia, ophiolite, ophiologist, ophiology, ophiophagous, Ophiophagus, ophiophagy, ophiophobia, Ophisaurus, Ophisops, ophitic |
| ophthalmós | | eye | ophthalm-[157] | ὀφθαλμός (ophthalmós) | exophthalmic, exophthalmos, microphthalmia, ophthalmia, ophthalmic, ophthalmologic, ophthalmologist, ophthalmology, ophthalmoparesis, ophthalmoplegia, parophthalmia, xerophthalmia |
| ópisthen |  | behind | opisth-[158] | ὄπισθεν (ópisthen), ὀπίσθιος | anopisthograph, opisthion, opisthobranch, opisthodomos, opisthoglyphous, opisthognathous, opisthograph, opisthokont, opisthosoma, opsimath, opsimathy |
| ópsesthai | | eye | ops-, opt- (ΟΠ) | ὄψεσθαι (ópsesthai), ὀπτός (optós), ὀπτικός (optikós), ὄψις (ópsis) | amblyopia, anopia, autopsy, biopsy, catadioptrics, catoptrics, catoptromancy, catoptrophobia, cyclops, diopter, dioptre, dioptrics, diplopia, eisoptrophobia, emmetropia, hemianopsia, myopia, opsoclonus, optic, optokinetic, panopticon, pleoptics, synopsis, synoptic, tritanopia |
| opsé |  | late | opsi-[159] | ὀψέ (opsé), ὄψιος | opsimath |
| opsōneîn |  | cook, prepare for eating | opson-[160] | ὀψωνεῖν (opsōneîn), (opsōnia) | opsonin, opsonoid |
| óros |  | mountain | or-, oro-[161] | ὄρος, ὄρεος (óros, óreos), ὀρειάς | Oread, orogenesis, orogenic, orogeny, orographic, orography, oronym |
| órkhis |  | testicle | orch-[162] | ὄρχις (órkhis), ὀρχίδιον (orkhídion) | anorchia, cryptorchidism, monorchism, orchid, orchiectomy, orchiopexy, Orchis, polyorchidism |
| orkheîsthai |  | dance | orches-[163] | ὀρχεῖσθαι (orkheîsthai) | orchestra |
| orégein |  | reach | oreg-[164] | ὀρέγειν (orégein), ὀρεκτός (orektós), ὀρεκτικός (orektikós), ὄρεξις (órexis), ὄρεγμα (óregma) | anorectic, anorexia, dysorexia, orectic, orexin, parorexia |
| órgia |  | work | org- | ὄργια (órgia) | orgasm |
| órganon |  | organ, instrument, tool | organ- | ὄργανον (órganon) | organic, organism, organogenesis |
| órnis |  | bird | ornith-[165] | ὄρνις, ὄρνιθος (órnis, órnithos) | Avernus, ornithology, ornithomancy, ornithorhynchus, ornithosis |
| orphanós | | | orphan-[166] | ὀρφανός (orphanós), ὀρφανότης | orphan |
| orthós |  | straight | orth-[167] | ὀρθός (orthós), ὀρθότης | orthocenter, orthocentric, orthodontia, orthodontic, orthodontist, orthodox, orthodoxy, orthographic, ortholog, orthologous, orthonym, orthopedic, orthoscope, orthosis, orthostat, orthostyle, orthotic |
| óruza |  | rice | oryz-[168] | ὄρυζα (óruza), ὀρύζιον | Oryza, Oryzomys, rice, risotto |
| osmḗ |  | odor | osm-[169] | ὀσμή (osmḗ), ὄσμησις | anosmia, anosmic, cacosmia, coprosmia, dysosmia, dysosmic, euosmia, hyperosmia, hyperosmic, hyposmia, hyposmic, osmium, osmolagnia, osmophobia, osphresiolagnia, parosmia, phantosmia, troposmia |
| ōsmós |  | push, thrust | osm-[170] | ὠσμός (ōsmós) | osmometry, osmosis, osmostat, osmotic |
| ostéon |  | bone | oste-[171] | ὀστέον, ὀστέου (ostéon, ostéou), ὀστοῦν, ὀστοῦ (ostoûn, ostoû) | dysostosis, endosteum, exostosis, hyperostosis, monostotic, Osteichthyes, osteoarthritis, osteoblast, osteochondritis, osteochondrosis, osteoclast, osteogenic, osteogenesis, osteoid, osteology, osteolysis, osteoma, osteomalacia, osteonecrosis, osteopathy, osteopenia, osteoporosis, osteosarcoma, osteosis, osteothrombosis, osteotome, osteotomy, periosteum, synostosis |
| óstreion |  | shell | ostrac- | ὄστρειον (óstreion), ὀστρακίζω (ostrakízō), ὄστρακον (óstrakon) | Entomostraca, Leptostraca, Malacostraca, ostracism, ostracize, ostracod, ostracoderm, ostracon, periostracum |
| |  | oyster | ostre-[172] | ὄστρεον | ostreophagist, oyster |
| oûs |  | ear | ot-[173] | οὖς, ὠτός (oûs, ōtós) | anotia, Aotus, microtia, Myosotis, otalgia, otic, otitis, otocephaly, otocleisis, otoconium, otocyst, otodynia, otolith, otology, otopathy, otophyma, otoplasty, otorhinology, otorrhea, otosclerosis, otoscope, otoscopy, ototomy, parotic, parotid, periotic, synotia |
| oxús |  | sharp, pointed | oxy-[174] | ὀξύς (oxús) | anoxia, anoxic, dioxide, hypoxia, monoxide, oxide, oxyanion, oxygen, oxyhalide, oxymoron, oxyntic, oxytone, paroxysm, pentoxide, polyoxide, tetraoxygen, tetroxide, trioxide |
| ózein |  | smell | oz-[175] | ὄζειν (ózein), ὄζων (ózōn), ὄζη | ozocerite, ozone, ozopore |
