o

oak < ^{middle english} oke < ^{old english} ac / ^{old norse} eik (= oak tree) < ^{proto-germanic} *aiks {> ^swedish ek, ^danish eg, ^dutch eik, ^german Eiche}.

oar < ^{middle english} or < ^{old english} ar < ^{proto-germanic} *airo.

oasis < ^french oasis < ^{late latin} oasis < ^{ancient greek} όασις | oassis < ^egyptian wh’t (= pit, depression).

oath < ^{middle english} oth < ^{old english} að < ^{proto-germanic} *aithaz < ^I.E. *oi-to- (= an oath).

obedience < ^{middle english} obedience < ^{old french} obedience (= submission) < ^latin oboedientia (= obedience) < oboediens [gen. obedientis], present participle of ob(o)edo [inf. ob(o)edire] (= listen to, obey) < ob (= to, over) + audio [inf. audire] (= listen, hear).

obelisk < ^{middle french} obélisque < ^latin obeliscus < ^{ancient greek} οβελίσκος | obeliskos (= small spit, obelisk, leg of a compass), a diminutive of οβελός | obelos (= a spit, pointed pillar, obelisk).

obese < ^latin obesus (= fat, stout, plump) < obesus, past participle of obedo [inf. obedere] (= eat all over, devour) < ob (= to, over) + edo [inf. edere] (= eat).

obey < ^{middle english} obeien < ^{old french} obeir (= obey) < ^latin ob(o)edo [inf. ob(o)edire] (= listen to, obey) < ob (= to, over) + audio [inf. audire] (= listen, hear).

obituary < ^{medieval latin} obituarius (= a record of one’s death) < ^latin obitus (= departure) [an euphemism for death] < obeo [inf. obire] (= go toward) < ob (= to, over) + eo [inf. ire] (= go).

object < ^{middle english} object < ^{old french} object < ^{medieval latin} objectum (= thing put before) < ^latin objectum, of objectus (= lying before, opposite), neuter past participle of objicio [inf. objicere] (= to present, oppose) < ob (= to, over) + jacio [inf. jacere] (= to throw).

oblige < ^{middle english} obligen < ^{old french} obligier (= commit, pledge) < ^latin obligo [inf. obligare] (= to bind, put under obligation) < ob (= to, over) + ligo [inf. ligare] (= to bind).

oblique < ^{middle english} oblique < ^{old french} oblique < ^latin obliquus (= slanting, sidelong, indirect) < (?) ob (= to, over) + licinus (= bent upward).

oblivion < ^{middle english} oblivioun (= lost of memory) < ^{old french} oblivion < ^latin oblivio (= forgetfulness) < obliviscor [inf. oblivisci] (= forget) < (?) ob (= to, over) + levis (= smooth).

oboe < ^italian oboe < ^french hautbois < haut (= high, loud) + bois (= wood).

obscure < ^{middle english} obscure < ^{old french} o(b)scur (= dark, clouded) < ^latin obscurus (= dark, dusky, unknown) < ob (= to, over) + -scurus (= covered).

observe < ^{middle english} observen < ^{old french} observer / osserver (= watch over, follow) < ^latin observo [inf. observare] (= watch over, note, guard) < ob (= to, over) + servo [inf. servare] (= watch, keep safe).

obsess < ^{middle english} obsess < ^latin obsessus, past participle of obsideo [inf. obsidere] (= sit opposite to, watch closely, besiege) < ob (= to, over) + sedeo [inf. sedere] (= to sit).

obstacle < ^{middle english} obstacle < ^{old french} o(b)stacle (= opposition, obstruction) < ^latin obstaculum (= hindrance, obstacle) < obsto [inf. obstare] (= stand before, hinder) < ob (= to, over) + sto [inf. stare] (= to stand).

obtain < ^{middle english} obteinen < ^{old french} obtenir (= acquire, obtain) < ^latin obtineo [inf. obtinere] (= hold, hold fast, acquire) < ob (= to, over) + teneo [inf. tenere] (= to hold).

obvious < ^{middle english} obvious < ^latin obvius (= open, exposed) < obviam (= in the way) < ob (= to, over) + viam, accusative of via (= way, road).

occasion < ^{middle english} occasioun < ^{old french} ochaison / ocasion (= reason, excuse, opportunity) < occasio (= opportunity, appropriate time, cause) < occasus, past participle of occido [inf. occidere] (= fall down, go down) < ob (= to, over) + cado [inf. cadere] (= to fall).

occupy < ^{middle english} occupien < ^{old french} oc(c)uper (= occupy, hold, seize) < ^latin occupo [inf. occupare] (= take over, seize, possess) < ob (= to, over) + capio [inf. capere] (= take, hold).

occur < ^{middle french} occurrer (= happen unexpectedly) < ^latin occuro [inf. occurrere] (= run to meet, run against) < ob (= to, over) + curro [inf. currere] (= to run).

ocean < ^{middle english} occean < ^{old french} occean < ^latin oceanus < ^{ancient greek} ωκεανός | ōkeanos (= the great river surrounding the Earth).

Oceania < ^{modern latin} Oceania < ^french Océanie < ^{old french} occean < ^latin oceanus < ^{ancient greek} ωκεανός | ōkeanos (= the great river surrounding the Earth).

ochre < ^{middle english} ocre / oker < ^{old french} ocre < ^{medieval latin} ocra < ^latin ochra < ^{ancient greek} ώχρα | ōkhra < ωχρός | ōkhros (= pale yellow).

octave < ^{medieval latin} octava < ^latin octava [dies] (= eighth [day]) < octava, female of octavus (= eighth) < octo (= eight) {> ^italian otto, ^spanish ocho, ^french huit} < ^I.E. *octo(u)- (= eight) {> ^{ancient greek} οκτώ | oktō, ^sanskrit astau}.

Octavian < ^latin Octavian < Octavius < octavus (= eighth [child in birth order]) < octo (= eight) < ^I.E. *octo(u)- (= eight).

October < ^{old english} October < ^latin October [mensis] (= eight [month]) [of the old Roman calendar, which began with March] < octo (= eight) + [suffix] -ber (< -bris).

octopus < ^{ancient greek} οκτόπους | oktōpous < οκτώ | oktō (= eight) + πους | pous (= foot).

odd < ^{middle english} odde < ^{old norse} oddi (= third or additional number, angle) < ^{proto-germanic} *uzdaz (= pointed upward).

ode < ^{middle french} ode < ^{late latin} ode (= lyric song) < ^{ancient greek} ω_ιδή | ō_idē < αοιδή | aoedē (= song, ode) < αείδω | aoedō (= to sing).

odeon < ^{ancient greek} ωδείον | ōdoeon (= building for musical performance) < ^{ancient greek} ω_ιδή | ō_idē (= song, ode) < αοιδή | aoedē (= song, ode) < αείδω | aoedō (= to sing).

odor < ^{middle english} odor < ^anglo-french odour < ^{old french} odor (= smell, perfume, fragrance) < ^latin odor (= a smell, scent) < ^I.E. *hed- (= to smell).

odyssey < ^latin Odyssēa < ^{ancient greek} Οδύσσεια | Odyssoea [the second poem of Homer, describing the adventures of Odysseus after the fall of Troy (described in his first poem, Iliad)] < ^{ancient greek} Οδυσσεύς | Odyssefs (= Odysseus, Ulisses), king of Ithaca island.

of < ^{old english} of < æf (= away, away from) < ^{proto-germanic} *af < ^I.E. *apo- (= off, away).

off < ^{middle english} off < ^{old english} of < æf (= away, away from) < ^{proto-germanic} *af < ^I.E. *apo- (= off, away).

offend < ^{middle english} offenden < ^{old french} ofendre (= hit, attack) < ^latin offendo [inf. offendere] (= hit, thrust, strike against) < ob (= to, over) + -fendo [inf. -fendere] (= to strike).

offer < ^{middle english} offeren < ^{old english} ofrian (= bring forward, show, exhibit) < ^latin offero [inf. offerre] (= bring before, present, bestow) < ob (= to, over) + fero [inf. ferre] (= bring, carry).

office < ^{middle english} office < ^anglo-french / ^{old french} ofice (= place, function, divine service) < ^latin officium (= service, function, kindness, official duty) < opificium (= work-doing) < ops [gen. opis] (= power, might) + facio [inf. facere] (= make, do).

often < ^{middle english} often < ^{old english} oft (= repeatedly, frequently) < ^{proto-germanic} *ufta- (= frequently) < (?) ^I.E. *upo (= under).

oil < ^{middle english} oil < ^anglo-french / ^{old french} olie < ^{old french} oile / uile (= oil) < ^latin oleum (= olive oil) < ^{ancient greek} έλαιον | elaeon (= olive oil) < ελαία | elaea (= olive tree, olive).

ointment < ^{middle english} oynement < ^{old french} oignement (= ointment, salve, unguent) < ^{vulgar latin} *unguimentum < ^latin unguentum (= ointment) < unguo [inf. unguere] (= to anoint with ointment).

o.k. < (?) an abbreviation [with jocular misspelling] of oll korrect (= all correct), in Boston and New York [1839], as an election slogan by the boosters of Martin Van Buren, in allusion to his nickname Old Kinderhook (< Kinderhook, a village of N.Y., where he was born).

Olaf < ^{old norse} An-leifr (= ancestor’s relic) < ^{old high german} ano (= ancestor) + leifr, related to ^{old english} læfan (= to leave).

old < ^{old english} ald / eald (= antique, ancient, elder, mature) < ^{proto-germanic} *althaz (= grown up, adult) < ^I.E. *al-₍₂₎ (= grow, nourish).

Oleg < ^russian Oleg < ^norse Helge < Helgi (= holy) < ^{proto-germanic} *hailaga < ^I.E. *kailo- (= whole, uninjured).

Olga < ^russian Olga < ^norse Helga (= holy) < ^{proto-germanic} *hailaga < ^I.E. *kailo- (= whole, uninjured).

oligarchy < ^{middle french} oligarchie < ^{ancient greek} ολιγαρχία | oligarkhia (= government by the few) < ολίγος | oligos (= small, few) + άρχω | arkhō (= to rule).

olive < ^{middle english} olive < ^{old french} olive < ^latin oliva (= olive, olive tree) < ^{ancient greek} ελαία | elaea (= olive, olive tree).

Oliver < ^{middle english} Oliver < ^french Olivier [by influence of olive (= olive tree)] < ^{middle low german} Alfihar (= elf-host, elf-army) < alf (= elf) + hari (= host, army).

Olivia < ^italian Olivia < ^latin oliva (= olive, olive tree) < ^{ancient greek} ελαία | elaea (= olive, olive tree).

Olympic < ^{ancient greek} Ολυμπικός | Olympikos < 1. Όλυμπος | Olympos, a mountain [home of the twelve great gods] or 2. Ολυμπιακός | Olympiakos < Ολυμπία | Olympia, a town [with a famous temple of Zeus, where every four years athletic contests were held in his honor from 776 B.C.].

omega < ^{middle english} omega < ^{medieval greek} ωμέγα | ōmega < ^{ancient greek} ω μέγα | ō mega (= big “o”) [long o in pronunciation, in contrast to όμικρον | omicron (= small “o”)] < [letter] ω + μέγα, neuter of μέγας | megas (= big).

omelet < ^french omelette < alemette < alemele (= blade, omelet) < (?) la lemelle < ^latin lamella (= thin plate), a diminutive of lamina (= plate, layer).

omen < ^latin omen [gen. ominis] (= foreboding, augury) (< ^{old latin} osmen).

omerta < ^italian omertà < [dialectal] umilta (= humility) [in reference to submission of its members] < ^latin humilitas (= lowness) < humilis (= on the ground, humble) < humus (= earth).

omg < [1994] an abbreviation of oh my God.

omicron < ^{middle english} omicron < ^{medieval greek} όμικρον < ^{ancient greek} ο μικρόν (= small “o”) [short o in pronunciation, in contrast to ωμέγα | ōmega (= big “o”)] < [letter] o + μικρόν, neuter of μικρός | mikros (= small).

omit < ^{middle english} omitten < ^latin omitto [inf. omittere] (= let go, let fall) < ob (= to, over) + mitto [inf. mittere] (= let go, send).

omnibus < ^french [voiture] omnibus (= [carriage] for all) {> ^english bus} < ^latin omnibus, dative plural of omnis (= all).

omnipotence < ^{middle english} omnipotens < ^{old french} omnipotence < ^{late latin} omnipotentia (= almighty power) < ^latin omnipotens (= all-powerful, almighty) < omnis (= all) + potens [gen. potentis] (= powerful).

on < ^{old english} on < an (= in, on, into) < ^{proto-germanic} *ana (= on) < ^I.E. *an-₍₁₎ (= on).

once < ^{middle english} anes < one + [adverbial genitive] -s.

one < ^{middle english} one < ^{old english} an (= one) < ^{proto-germanic} *ainaz {> ^{old norse} einn, ^danish een, ^dutch een, ^gothic ains, ^german ein} < ^I.E. *oi-no- (= one, unique) {> ^latin unus}.

oneiric < ^{ancient greek} όνειρος, όνειρον | onoeros, onoeron (= a dream).

onion < ^{middle english} oinyon / ungeon / unione < ^anglo-french union / ^{old french} oignon (= onion) < ^latin unio (= unity, a kind of onion [in a string (a twist) of onions]) < unus (= one, unique).

only < ^{middle english} onli < ^{old english} ænlic / anlic (= unique, solitary) < an (= one) + -lic (= -like).

onomatology < ^{ancient greek} όνομα | onoma [gen. ονόματος | onomatos) (= name) + -λογία | -logia (= study) (< λόγος | logos (= a speaking) < λέγω | legō (= speak)).

ontology < ^{modern latin} ontologia < ^{ancient greek} ον | on [gen. όντος | ontos], neuter present participle of ειμί | oemi (= I am) + -λογία | -logia (= study) (< λόγος | logos (= a speaking) < λέγω | legō (= speak)).

onyx < ^{middle english} oneche / onix < ^{old french} oniche < ^latin onyx < ^{ancient greek} όνυξ | onyx (= claw, nail, onyx-stone).

oology < ^{ancient greek} ωόν | ōon (= egg) + -λογία | -logia (= study) (< λόγος | logos (= a speaking) < λέγω | legō (= speak)).

opacity < ^french opacité < ^latin opacitas (= shade, shadiness) < opacus (= shaded, dark).

opal < ^{middle french} opalle < ^{late latin} opalus < ^sanskrit upala-s (= gem, precious stone).

opaque < ^{middle english} opake < ^latin opacus (= shady, dark, obscure).

open < ^{old english} open < ^{proto-germanic} *upana- (= put up, set up) {> ^{old norse} opinn, ^german offen} < ^I.E. *upo (= under, over).

opera < ^italian opera (= work, composition) < ^latin opera (= work, effort) < operor [inf. operari] (= to work) < opus [gen. operis] (= a work) < ^I.E. *op- (= to work).

operation < ^{middle english} operation < ^{old french} operacion (= operation, working, proceedings) < ^latin operatio (= a working, operation) < operor [inf. operari] (= to work) < opus [gen. operis] (= a work).

Ophelia < ^{ancient greek} ωφέλεια | ōpheloea (= help, aid) < ωφελώ | ōphelō (= to help, aid) < όφελος | ophelos (= advantage, help) < ^I.E. *obhel- (= to avail).

ophidian < ^{ancient greek} οφίδιον | ophidion, a diminutive of όφις | ophis (= serpent, snake).

Ophiuchus < ^latin ophiuchus < ^{ancient greek} οφιούχος | ophioukhos (= holding a serpent) < όφις | ophis (= serpent, snake) + έχω | ekhō (= hold, have).

ophthalmic < ^latin ophthalmicus < ^{ancient greek} οφθαλμικός | ophthalmikos (= of the eyes) < οφθαλμός | ophthalmos (= eye).

opinion < ^{middle english} opinioun < ^{old french} opinion (= opinion, view) < ^latin opinio (= opinion, conjecture, belief) < opinor [inf. opinari] (= think, judge, suppose).

opium < ^{middle english} opium < ^latin opium < ^{ancient greek} όπιον | opion (= poppy juice, poppy), a diminutive of οπός | opos (= plant juice).

opponent < ^latin opponens [gen. oponentis], present participle of oppono [inf. opponere] (= set opposite, oppose) < ob (= to, over) + pono [inf. ponere] (= to put, place).

opportune < ^{middle english} opportune < ^{old french} opportune < ^latin opportunus (= fit, convenient, favorable) < [ventus] ob portum [veniens] (= [wind] toward a port [coming]) < ob (= to, over) + portus (= port, harbor).

opposite < ^{middle english} opposite < ^{old french} op(p)osite (= contrary) < ^latin oppositus (= opposite), past participle of oppono [inf. opponere] (= set opposite, oppose) < ob (= to, over) + pono [inf. ponere] (= to put, place).

optic < ^{middle english} optik < ^{old french} obtique / optique < ^{medieval latin} opticus (= of seeing) < ^{ancient greek} οπτικός | optikos (= of seeing) < οπτός | optos (= visible) < stem οπ- | op- < ^I.E. *okw- (= to see).

optimum < ^latin optimum, neuter of optimus (= best), superlative of bonus (= good).

option < ^{middle english} option < ^french option < ^latin optio (= choice, liberty to choose) < opto [inf. optare] (= to desire, choose).

opus < ^latin opus [gen. operis] (= work, labor, exertion) {> ^spanish obra, ^italian opera, ^french oeuvre} < ^proto-italic *opes- (= work) < ^I.E. *op- (= to work).

or < ^{middle english} or (= either, else, otherwise) < ^{old english} oþþe (= either, or).

oracle ^{middle english} oracle < ^{old french} oracle (= temple, oracle) < ^latin orac(u)lum (= divine announcement, oracle) < oro [inf. orare] (= to pray, beseech).

oral < ^{late latin} oralis (= of the mouth) < ^latin os [gen. oris] (= mouth, opening) < ^I.E. *os- (= mouth).

orange < ^{middle english} orange < ^{old french} orange / orenge < ^{medieval latin} orenge < ^italian arancia < narancia < ^arabic naranj < ^persian narang < ^sanskrit naranga-s (= orange tree).

orangutan < ^french orang-ou-tang < ^dutch orang-outang < ^malay orang utan (= man of the forest) < orang (= man) + (h)utan (= forest, wilderness).

oratorio < ^italian oratorio < ^{late latin} oratorium (= a place of prayer, oratory, chapel) < oratorium, neuter of ^latin oratorius (= of praying) < oro [inf. orare] (= to pray, beseech).

orbit < ^{middle english} orbit < ^{old french} orbite < ^latin orbita (= wheel track, path, course).

orca < orc / ork < ^french orque < ^latin orca (= cetacean, a kind of whale).

orchard < ^{old english} orceard (= fruit garden) < ortgeard < (?) wortgeard < wyrt (= vegetable, root)) + geard (= garden).

orchestra < ^{middle english} orchestra < ^latin orchestra < ^{ancient greek} ορχήστρα | orkhēstra (= the place in a theater for the chorus of dancers) + ορχούμαι | orkhoumae (= to dance).

orchid < ^{modern latin} orchideæ < ^latin orchis, a kind of orchid < ^{ancient greek} όρχις | orkhis (= testicle) [because the root of this plant looks like testicle].

order < ^{middle english} order < ^{old french} ordre (= position, estate, rule) < ordene < ^latin ordo [gen. ordinis] (= row, rank, series, arrangement).

ordinary < ^{middle english} ordinarie < ^{old french} ordinarie (= usual, ordinary) < ^latin ordinarius (= customary, regular, usual) < ordo [gen. ordinis] (= row, rank, series, arrangement).

ore < ^{middle english} ore < a merge of ^{old english} ora (= ore, unworked metal) + ^{old english} ar (= brass, copper, bronze) (< ^{proto-germanic} *ajiz- < ^I.E. *aus-₍₂₎ (= gold)).

oregano < ^{american spanish} / ^spanish oregano < ^latin origanus / origanum < ^{ancient greek} ορείγανον | oroeganon < όρος | oros (= mountain) + γάνος | ganos (= brightness, ornament).

organ < [^{old english} organe / ^{old french} orgene] (= musical instrument) < ^latin organum (= organ, musical instrument) < ^{ancient greek} όργανον | organon (= tool, musical instrument, organ of the body).

organization < ^{middle english} organisacioun (= structure of the body, act of organizing) < ^{medieval latin} organizatio (= an organizing) < organizo [inf. organizare] (= organize) < ^latin organum (= organ, musical instrument) < ^{ancient greek} όργανον | organon (= tool, musical instrument, organ of the body).

orgasm < ^{modern latin} orgasmus / ^french orgasme < ^{ancient greek} οργασμός | orgasmos (= excitement, swelling) < οργώ | orgō (= swell, be excited, be in heat) < οργή | orgē (= impulse, excitement, anger).

orgy < orgies [plural] < ^{middle french} orgies < ^latin orgia < ^{ancient greek} όργια | orgia [plural] (= secret rites for a god, especially Dionysus).

oriel < ^{middle english} oriel < ^{old french} oriol (= hall, vestibule) / ^{medieval latin} oriolum (= porch, small room, gallery) < (?) ^{vulgar latin} *auraeolum < aulaeolum < a diminutive of ^latin aulaeum (= curtain).

orient < ^{middle english} orient < ^{old french} orient (= east) < ^latin oriens [gen. orientis] (= the rising sun, the east) < oriens, present participle of orior [inf. oriri] (= to rise).

origami < ^japanese origami < ori (= fold) + kami (= paper).

origin < ^{middle english} origin (= ancestry, race) < ^latin origo [gen. originis] (= a rise, beginning, source, descent, birth) < orior [inf. oriri] (= to rise, be born, start).

Orlando < ^italian Orlando < ^french Roland < ^{old high german} Hrodland < hrod (= famous) + land).

Orleans < ^french Orléans < ^latin Aurelianum < Aurelian, an emperor.

ornament < ^{middle english} ournement < ^{old french} ornement (= decoration), < ^latin ornamentum (= apparatus, equipment, decoration) < orno [inf. ornare] (= to equip, adorn) < ordo (= row, rank, series, arrangement).

ornithology < ^{modern latin} ornithologia < ^{ancient greek} όρνις | ornis [gen. όρνιθος | ornithos] (= bird) + -λογία | -logia (= study) (< λόγος | logos (= a speaking) < λέγω | legō (= speak)).

orphan < ^{middle english} orphan < ^{late latin} orphanus (= parentless child) < ^{ancient greek} ορφανός | orphanos (= orphaned, without parents) < *όρφος | *orphos (= bereft).

orthodox < ^{middle english} orthodox < ^{late latin} orthodoxus < ^{ancient greek} ορθόδοξος | orthodoxos (= with the right opinion) < ορθός | orthos (= right, true, correct) + δόξα | doxa (= opinion) (< δοκώ | dokō (= seem, believe)).

orthopedic < ^french orthopédique < orthopédie < ^{ancient greek} ορθός | orthos (= right, straight, correct) + παις | paes [gen. παιδός | paedos) (= child).

Osbo(u)rn(e) < ^scandinavian Osbo(u)rn(e) (= god-bear) < os (= god) + ^{proto-germanic} *beranan (= bear).

Oscar < ^{old english} Osgar (= god’s spear) < os (= god) + gar (= spear).

oscillation < ^french oscillation < ^latin oscillatio (= a swinging) < oscillo [inf. oscillare] (= to swing) < oscillum (= a swing) < (?) ob(s) (= to, over) + cillo (= move).

Oslo < ^norwegian Oslo < (?) 1. ^{old norse} os (= estuary, river mouth) or 2. ^{old norse} As, a god + Lo, a river.

Osmond < ^{old english} Osmund < os (= god) + -mund (= protection).

osmosis < osmose / osmosis, a shortening of endosmose / endosmosis < ^{ancient greek} ένδον | endon (= inward) + ωσμός | ōsmos (= a pushing) (< ωθώ | ōthō (= push)).

ossification < ^latin os [gen. ossis] (= bone) + -fication (= a making or causing) (< facio [inf. facere] (= make, do)).

ostentation < ^{middle english} ostentacioun < ^{old french} ostentacion < ^latin ostentatio (= exhibition, vain display) < ostento [inf. ostentare] (= to display) < ostendo [inf. ostendere] (= to show, exhibit, display) < ob (= to, over) + tendo [inf. tendere] (= to stretch).

osteoporosis < ^{ancient greek} όστεον | osteon (= bone) + πόρος | poros (= passage, pore).

ostracism < ^{middle french} ostracisme < ^{ancient greek} οστρακισμός | ostrakismos (= ostracism) < οστρακίζω | ostrakizō (= to ostracize) < όστρακον | ostrakon (= potsherd) [because in Athens used to write the name of whom wanted to vote for exile upon a potsherd (a material for writing, easy to find it in the streets from the broken vessels)].

ostrich < ^{middle english} ostrich / hostriche / estrich < ^{old french} ostruce / ^{medieval latin} ostrica / ostrigius < ^{vulgar latin} avis struthio < ^latin avis (= bird) + ^{late latin} struthio (= ostrich) (< ^{ancient greek} στρουθίον | strouthion (= little sparrow, ostrich) < στρουθός [μεγάλη] | strouthos [megalē] (= [big] sparrow)).

Oswald < ^{old english} Osweald (= god-power) < ^{old english} os (= god) + ^{old english} (ge)weald (= power).

other < ^{old english} oþer (= the second of two, one of the two, additional) < ^{proto-germanic} *anthera- {> ^{old norse} annarr, ^dutch ander, ^german ander}.

Otto < ^german Otto < ^{old high german} Odo / Udo < ^{proto-germanic} *aud- (= wealth).

ouch < ^german outch, cry of pain < autsch.

ought < ^{old english} ahte (= owned, possessed), past tense of agan (= to own, possess).

ounce < ^{middle english} ounce < ^{old french} once / unce < ^latin uncial (= one-twelfth part [of a pound]) < ^latin unus (= one) [the abbreviation oz. is from ^{older italian} onza].

our < ^{old english} ure (= of us, pertaining to us) [genitive plural] < ^{proto-germanic} *ons < ^I.E. *nes- (= we) {> ^latin nos}.

out < ^{old english} ut (= out, without) < ^{proto-germanic} *ūt- < ^I.E. *uidh- (= up, out, on high).

outrage < ^{middle english} outrage < ^{old french} outrage (= harm, damage) < oltrage < ^{vulgar latin} *ultraticum (= excess) < ^latin ultra (= beyond).

oval < ^{middle french} ovalle (= oval figure) < ^{medieval latin} ovalis (= of eggs) < ^latin ovum (= egg) < ^I.E. *ōwyo‑ / *ōyyo‑ (= egg).

ovary < ^{modern latin} ovarium (= ovary) < ^{medieval latin} ovaria (= the ovary of a bird) < ^latin ovum (= egg) < ^I.E. *ōwyo‑ / *ōyyo‑ (= egg).

ovation < ^{middle french} ovation < ^latin ovatio (= a triumph, rejoicing) < ovo [inf. ovare] (= to triumph, rejoice, exult) < (?) imitative origin.

oven < ^{old english} ofen (= furnace) < ^{proto-germanic} *ukhnaz {> ^{old norse} ofn, ^dutch oven, ^german Ofen} < ^I.E. *aukw- (= cooking pot).

over < ^{old english} ofer (= beyond, above, upon) < ^{proto-germanic} *uberi {> ^{old norse} yfir, ^german über) < ^I.E. *uper (= over) {> ^{ancient greek} υπέρ | yper}.

overt < ^{middle english} overt (= open, unfastened) < ^{old french} overt, past participle of ovrir (= to open) < ^latin aperio [inf. aperire] (= to open, uncover).

overture < ^{middle english} overture < ^{old french} overture (= opening, proposal) < ^latin apertura (= opening) < aperio [inf. aperire] (= to open, uncover).

owe < ^{old english} agan (= have, own) < ^{proto-germanic} *aiganan (= to possess) < ^I.E. *aik- (= to possess).

Owen < ^celtic Owen < ^gaelic Eòghann / ^{old irish} Eogán / ^{old welsh} Eugein, Ougein < ^latin Eugenius < ^{ancient greek} Ευγένιος | Evgenios < ευγενής | evgenēs (= nobleman, gentle) < ευ | ef, ev (= well) + γένος | genos (= parentage).

owl < ^{middle english} oule < ^{old english} ule (= owl) < ^{proto-germanic} *uwwalon- < ^I.E. *u(wa)l-, of imitating origin.

own < ^{middle english} ahnen / ounen (= to possess, have) < ^{old english} geagnian < agan (= have, own) < ^{proto-germanic} *aiganan (= to possess) < ^I.E. *aik- (= to possess).

own < ^{middle english} ouen < ^{old english} agen (= possessed by) < ^{proto-germanic} *aiganaz (= possessed, owned) < ^I.E. *aik- (= to possess).

ox < ^{middle english} oxe < ^{old english} oxa (= ox) < ^{proto-germanic} *ukhson {> ^{old norse} oxi, ^{middle dutch} osse, ^gothic auhsa, ^german Ochse} < ^I.E. *uks-en- (= male animal) {> ^welsh ych, ^{middle irish} oss, ^avestan uxshan-, ^sanskrit uksa}.

Oxford < ^{middle english} Oxforde < ^{old english} Oxnaforda (= oxen ford) < ox + ford.

oxide < ^french oxide < oxygène (= oxygen) + acide (= acid).

oxygen < ^french oxygène < ^{ancient greek} οξύς | oxys (= sharp, acid) + stem γεν- | gen-, γον- | gon- of γίγνομαι | gignomae (= become, produce) [so called because joined with elements produces acid].

oxymoron < ^{ancient greek} οξύμωρον | oxymōron, neuter of οξύμωρος | oxymōros (= pointedly foolish) < οξύς | oxys (= sharp, pointed) + μωρός | mōros (= stupid).

oyster < ^{middle english} oistre < ^{old french} oistre / uistre < ^latin ostreum (= oyster) < ^{ancient greek} όστρεον | ostreon (= oyster).

ozone < ^german Ozon < ^{ancient greek} όζον | ozon, neuter present participle of όζω | ozō (= to smell) [so called for its pungent odor].