Online Etymology Dictionary

olfactory (adj.): 1650s, from Latin olfactorius, from olfact-, past participle stem of olfacere "to get the smell of, sniff," from olere "emit a smell, give off a smell of" (see odor) + facere "make" (see factitious).
Olga: fem. proper name, from Russian, probably from Norse Helga, literally "holy," from Proto-Germanic *haliaga, from PIE *kailo- (see health). The masc. form is Oleg.
oligarch (n.): c.1600, from Middle French olygarche, oligarque, from Greek oligarkhes, related to oligarkhia (see oligarchy).
oligarchic (adj.): 1640s, from Greek oligarkhikos, from oligarkhos, related to oligarkhia (see oligarchy). Related: Oligarchical.
oligarchy (n.): 1570s, from Middle French oligarchie (14c.), from Greek oligarkhia "government by the few," from stem of oligos "few, small, little" (see oligo-) + arkhein "to rule" (see archon).
oligo-: before vowels olig-, word-forming element meaning "few, the few," from comb. form of Greek oligos "few, scanty, small, little," in plural, "the few;" of uncertain origin.
Oligocene (adj.): 1856, "pertaining to the Tertiary period between the Eocene and the Miocene," coined in German (1854) by Heinrich Ernst von Beyrich, from oligo- "small, little, few" + -cene. So called because few modern fossils were found in Oligocene rocks.
oligopolistic (adj.): 1939, from oligo- "little, small," in plural, "few" + -poly, from Greek polein "to sell" (see monopoly).
oligopoly (n.): 1887, from Medieval Latin oligopolium, from Greek oligos "little, small," in plural, "the few" (see oligo-) + polein "to sell" (see monopoly).
oligotrophy (n.): 1928, from oligo- "small, little" + -trophy "food, nourishment." Related: Oligotrophic.
oliguria (n.): 1843, from oligo- "small, little," + -uria, from Greek ouron "urine" (see urine).
olio (n.): medley dish of Iberian origin, 1640s, from Spanish olla, Portuguese olha, both from Vulgar Latin olla "pot, jar." Sense transferred to "any mixture or medley."
oliphant (n.): obsolete form of elephant, c.1200; also used in Middle English with sense "ivory horn."
olive (n.): c.1200, "olive tree," from Old French olive "olive, olive tree" (13c.) or directly from Latin oliva "olive, olive tree," from Greek elaia "olive tree, olive," probably from the same Aegean language (perhaps Cretan) as Armenian ewi "oil." Applied to the fruit or berry of the tree in English from late 14c. As a color from 17c. Olive branch as a token of peace is from early 13c.
Oliver: masc. personal name, in medieval lore the name of one of Charlemagne's peers, friend of Roland, from French Olivier, from Middle Low German Alfihar, literally "elf-host, elf-army," from alf "elf" (see elf) + hari "host, army" (see harry (v.)). Cognate with Anglo-Saxon name Ælfhere. Form influenced in Old French by olivier "olive tree."
Olivetti: brand of typewriters manufactured by company founded in 1908 near Turin, Italy; named for founder, Camillo Olivetti.
Olivia: fem. proper name, from Italian Olivia, from Latin oliva "olive" (see olive).
Olmec: 1787, from Nahuatl Olmecatl (plural Olmeca), literally "inhabitant of the rubber country."
Olympiad (n.): late 14c., "period of four years" (between Olympic games), from Old French olimpiade "period of four years," from Latin Olympiadem, from Greek olympiados, genitive of Olympias (see Olympic). Used by ancient Greeks as a unit in computing time. Revived in modern usage with revival of the games, 1896.
Olympian (adj.): "of or belonging to Olympus," c.1600; see Olympic + -ian. The noun meaning "a great god of ancient Greece" is attested from 1843, from Late Latin Olympianus, from Greek Olympios "pertaining to Olympus;" sense of "one who competes in the (modern) Olympic Games" is from 1976.
Olympic (adj.): c.1600, "of or in reference to Mount Olympos, also to Olympia (khora), town or district in Elis in ancient Greece, where athletic contests in honor of Olympian Zeus were held 776 B.C.E. and every four years thereafter; from Greek Olympikos, from Olympos, of unknown origin. The modern Olympic Games are a revival, begun in 1896. Not the same place as Mount Olympus, abode of the gods, which was in Thessaly.
Olympus: high mountain in Thessaly, abode of the gods, from Greek Olympos, of unknown origin. The name was given to several mountains, each seemingly the highest in its district.
om: mystical word in Hinduism, Buddhism; an utterance of assent, 1788.
Omaha: Siouan Indians of northeastern Nebraska, 1804, perhaps from Omaha umaha, perhaps literally "upstream (people), against the flow."
Oman: coastal nation in Arabia, supposedly named for its founder. Recorded from Roman times (Omana, in Pliny). Related: Omani.
ombre (n.): card game popular early 18c., from French hombre, from Spanish hombre "man" (see hombre). So called from an expression (translatable as "I am the man") spoken in the course of the game.
ombudsman (n.): 1959, from Swedish ombudsman, literally "commission man" (specifically in reference to the office of justitieombudsmannen, which hears and investigates complaints by individuals against abuses of the state); cognate with Old Norse umboðsmaðr, from umboð "commission" (from um- "around," see ambi-, + boð "command," see bid (v.)) + maðr "man" (see man (n.)).
omega: c.1400, from Medieval Greek omega, from classical Greek o mega "big 'o' " (in contrast to o micron "little 'o' "); so called because the vowel was long in ancient Greek. From mega (see mega-). The final letter of the Greek alphabet, hence used figuratively for "the last, final" of anything (cf. Rev. i:8),
omelet (n.): 1610s, from French omelette (16c.), metathesis of alemette (14c.), from alemele "omelet," literally "blade (of a knife or sword)," probably a misdivision of la lemelle (mistaken as l'alemelle), from Latin lamella "thin, small plate," diminutive of lamina "plate, layer" (see laminate). The food so called from its flat shape. The proverb "you can't make an omelet without breaking eggs" (1859) translates French On ne saurait faire une omelette sans casser des oeufs. Middle English had hanonei "fried onions mixed with scrambled eggs" (mid-15c.).
omen (n.): 1580s, from Latin omen "foreboding," from Old Latin osmen, of unknown origin; perhaps connected with the root of audire "to hear" [OED] or from PIE *o- "to believe, hold as true" (cf. Greek oiomai "I suppose, think, believe").
omer (n.): Hebrew measure of capacity (a little over 5 pints), from Hebrew 'omer.
omerta (n.): 1909, from dialectal form of Italian umilta "humility," in reference to the code of submission of individuals to the group interest, from Latin humilitas (see humility).
omg: Internet chat abbreviation of oh my God, by 1994. (Earlier in computerese it meant Object Management Group, 1989, a consortium which helped pave the way for the modern Internet.)
omicron: 15th letter of the Greek alphabet, literally "small 'o,' " from Greek (s)mikros "small," from PIE *smik-. Because the vowel was "short" in ancient Greek. Cf. Omega.
ominous (adj.): 1580s, from Latin ominosus "full of foreboding," from omen (genitive ominis) "foreboding" (see omen). Related: Ominousness.
ominously (adv.): 1590s, from ominous + -ly (2). In earliest use, "of good omen, auspicious;" meaning "of evil omen" first attested 1640s, in Milton.
omission (n.): late 14c., from Latin omissionem (nominative omissio) "an omitting," noun of action from past participle stem of omittere (see omit). Related: Omissible.
omit (v.): early 15c., from Latin omittere "let go, let fall," figuratively "lay aside, disregard," from assimilated form of ob (here perhaps intensive) + mittere "let go, send" (see mission). Related: Omitted; omitting.
omittance (n.): "omission," c.1600, perhaps coined by Shakespeare, who used it in wordplay ("Omittance is no quittance"), from omit + -ance.
omni-: word-forming element meaning "all," from Latin omni-, combining form of omnis "all, every, the whole, of every kind," of unknown origin, perhaps literally "abundant," from *op-ni-, from PIE root *op- "to work, produce in abundance" (see opus).
omnibus (n.): 1829, "four-wheeled public vehicle with seats for passengers," from French (voiture) omnibus "(carriage) for all, common (conveyance)," from Latin omnibus "for all," dative plural of omnis "all" (see omni-). Introduced by Jacques Lafitte in Paris in 1819 or '20, in London from 1829. In reference to legislation, the word is recorded from 1842. Meaning "man or boy who assists a waiter at a restaurant" is attested from 1888 (cf. busboy). As an adjective in English from 1842.
omnidirectional (adj.): 1927, from omni- + directional (see direction).
omnifarious (adj.): 1650s, from Late Latin omnifarius "of all sorts," from Latin omnifariam "on all places or parts," from omnis "all" (see omni-) + -fariam "parts" (cf. multifarious). Related: Omnifariously; omnifariousness.
omnipotence (n.): mid-15c., omnipotens, from Middle French omnipotence, from Late Latin omnipotentia "almighty power," from Latin omnipotentem "omnipotent" (see omnipotent). Related: Omnipotency (late 15c.).
omnipotent (adj.): early 14c., from Old French omnipotent "almighty, all-powerful" (11c.) or directly from Latin omnipotentem (nominative omnipotens) "all-powerful, almighty," from omnis "all" (see omni-) + potens (genitive potentis) "powerful" (see potent). Strictly only of God or a deity; general sense of "having absolute power or authority" is attested from 1590s.
omnipresence (n.): c.1600, from Medieval Latin omnipraesentia, from omnipraesens, from Latin omnis "all, every" (see omni-) + praesens "present" (see present (adj.)).
omnipresent (adj.): c.1600, from Medieval Latin omnipraesentem (nominative omnipraesens); see omnipresence. Related: Omnipresently.
omniscience (n.): 1610s, from Medieval Latin omniscientia "all-knowledge," from Latin omnis "all" (see omni-) + scientia "knowledge" (see science).
omniscient (adj.): c.1600, from Modern Latin omniscientem (nominative omnisciens), back-formation from Medieval Latin omniscientia (see omniscience). Related: Omnisciently.
omnisexual (adj.): by 1959, from omni- + sexual. Earliest application is to Walt Whitman.