Online Etymology Dictionary

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 (as in 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" (source also of 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. Compare 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- (1) "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 (compare 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" (compare 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.
omnium gatherum (n.): 1520s, "miscellaneous collection," humorous coinage from Latin omnium "of all" (genitive plural of omnis; see omni-) + Latinized form of English gather.
omnivore (n.): 1890, formed from omni- on model of carnivore (see omnivorous).
omnivorous (adj.): 1650s, from Latin omnivorus "all-devouring," from omnis "all" (see omni-) + vorare "devour, swallow" (see voracity). Related: Omnivorously; omnivorousness.
omo-: before vowels om-, word-forming element meaning "raw, unripe," from Greek omo-, comb. form of omos "raw," from PIE root *om- "raw, sharp-tasting" (source also of Sanskrit amah "raw, uncooked, unripe," Old Irish om, Welsh of).
omophagous (adj.): 1857, from omophagia (1706), from Greek, literally "eating raw flesh," from omos "raw" (see omo-) + phagein "to eat" (see -phagous).
omphalo-: before vowels omphal-, word-forming element meaning "navel," from Greek omphalos (see omphalos).
omphalos: also omphalus, "sacred stone," 1850, from Greek omphalos, literally "navel," later also "hub" (as the central point), from PIE *ombh-alo-, from root *nobh-/*ombh- "navel" (see navel). The name of the rounded stone in the shrine at Delphi, regarded by the ancients as the center of the world. Related: Omphalic.
omphaloskepsis (n.): 1925, from omphalo- + Greek -skepsis, from skeptesthai "to reflect, look, view" (see scope (n.1)). Also omphaloscopy (1931), and used in the sense of "navel-gazer" were omphalopsychic (1892), omphalopsychite (1882).
on (prep.): Old English on, unstressed variant of an "in, on, into," from Proto-Germanic *ana "on" (source also of Dutch aan, German an, Gothic ana "on, upon"), from PIE root *an- "on" (source also of Avestan ana "on," Greek ana "on, upon," Latin an-, Old Church Slavonic na, Lithuanian nuo "down from"). Also used in Old English in many places where we would now use in. From 16c.-18c. (and still in northern England dialect) often reduced to o'. Phrase on to "aware" is from 1877.
on-looker (n.): "spectator," c. 1600, from on + agent noun from look (v.).
on-site (adj.): also onsite, 1959, from on + site. Originally in reference to Cold War military inspections.
onager (n.): Asiatic wild ass, mid-14c., from Latin onager, from Greek onagros, from onos "ass" (related to Latin asinus, but the ultimate source is unknown) + agrios "wild," literally "living in the fields," from agros "field" (see acre).
onanism (n.): "masturbation," also "coitus interruptus," 1727, from Onan, son of Judah (Gen. xxxviii:9), who spilled his seed on the ground rather than impregnate his dead brother's wife: "And Onan knew that the seed should not be his; and it came to pass, when he went in unto his brother's wife, that he spilled it on the ground, lest that he should give seed to his brother." The moral of this verse was redirected by those who sought to suppress masturbation.
onboard (adj.): 1966 as one word, from on + board (n.2).
once (adv.): c. 1200, anes, from ane "one" (see one ) + adverbial genitive. Replaced Old English æne. Spelling changed as pronunciation shifted from two syllables to one after c. 1300. Pronunciation change to "wuns" parallels that of one. As an emphatic, meaning "once and for all," it is attested from c. 1300, but this now is regarded as a Pennsylvania German dialect formation. Meaning "in a past time" (but not necessarily just one time) is from mid-13c.

Once upon a time as the beginning of a story is recorded from 1590s. At once originally (early 13c.) meant "simultaneously," later "in one company" (c. 1300), and preserved the sense of "one" in the word; the phrase typically appeared as one word, atones; the modern meaning "immediately" is attested from 1530s.
once-over (n.): "inspection," 1913, from once + over.
onco-: word-forming element meaning "bulk, mass," especially in medical use, "tumor," from Latinized form of Greek onko-, comb. form of onkos "bulk, size, mass, body."
oncogene (n.): 1969, from onco- + -gene, from root of Greek gignere (perf. genui) "beget," from PIE *gen- "produce" (see genus). Related: Oncogenesis (1832).
oncology (n.): 1857, coined in English from onco- "tumor" + -logy "science or study of." Related: Oncologist; oncological.
oncoming (adj.): 1844, from on + coming, present participle of come (v.).