Online Etymology Dictionary

omnium gatherum: 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 *omo- (cf. 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" (cf. Dutch aan, German an, Gothic ana "on, upon"), from PIE root *an- "on" (cf. 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 time is from 1890.
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.).
one (n.): c.1200, from Old English an (adjective, pronoun, noun) "one," from Proto-Germanic *ainaz (cf. Old Norse einn, Danish een, Old Frisian an, Dutch een, German ein, Gothic ains), from PIE *oi-no- "one, unique" (cf. Greek oinos "ace (on dice);" Latin unus "one;" Old Persian aivam; Old Church Slavonic -inu, ino-; Lithuanian vienas; Old Irish oin; Breton un "one").

Originally pronounced as it still is in only, and in dialectal good 'un, young 'un, etc.; the now-standard pronunciation "wun" began c.14c. in southwest and west England (Tyndale, a Gloucester man, spells it won in his Bible translation), and it began to be general 18c. Use as indefinite pronoun influenced by unrelated French on and Latin homo.

One and only "sweetheart" is from 1906. One of those things "unpredictable occurrence" is from 1934. Slang one-arm bandit "a type of slot machine" is recorded by 1938. One-night stand is 1880 in performance sense; 1963 in sexual sense. One of the boys "ordinary amiable fellow" is from 1893. One-track mind is from 1927.
one-horse (adj.): "small-scale, petty" 1853, American English, colloquial, in reference to towns so small they only had one horse.
one-liner (n.): "short joke, witty remark," 1969, from one + line.
one-of-a-kind: adjectival phrase attested from 1961.
one-off (n.): "single example of a manufactured product," 1934, from one + off. Later given figurative extension.
one-shot (adj.): 1907, "achieved in a single attempt" (original reference is to golf), from one + shot (n.). Meaning "happening or of use only once" is from 1937.
one-sided (adj.): 1833, "dealing with one side of a question or dispute," from one + side (n.). Related: One-sidedly; one-sidedness.
one-upsmanship (n.): 1952, from noun phrase one up "scoring one more point than one's opponent" (1919).
one-way (adj.): 1906, in reference to travel tickets; 1914 in reference to streets; 1940 in reference to windows, mirrors, etc.; from one + way.
Oneida: Iroquois tribe of upper N.Y. state, who later moved in part to Wisconsin, 1666, named for its principal settlement, from Oneida onenyote', literally "erected stone," containing -neny- "stone" and -ot- "to stand."
oneiric (adj.): 1859, from Greek oneiros "a dream" (see oneiro-) + -ic.
oneiro-: before vowels oneir-, word-forming element meaning "dream," from Greek oneiros "a dream," from PIE *oner- "dream."
oneirocritic (n.): "a judge or interpreter of dreams," 1650s from Greek oneirokritikos "pertaining to the interpretation of dreams," from oneirokrites "interpreter of dreams," from oneiros "a dream" (see oneiro-) + krites "discerner, judge" (see critic).
oneirocritical (adj.): 1580s, from oneiro- + critic + -al (1).
oneiromancy (n.): 1650s; see oneiro- + -mancy. Cf. Greek oneiromantis "an interpreter of dreams." Related: oneiromantic.
oneness (n.): 1590s, from one + -ness. A re-formation of Middle English onnesse, which vanished by 13c.
onerous (adj.): late 14c., from Old French onereus, honereus (14c., Modern French onéreux) and directly from Latin onerosus, from onus (genitive oneris) "burden" (see onus).
oneself: 1540s, one's self. Hyphenated 18c.; written as one word from c.1827, on model of himself, itself, etc.
ongoing (adj.): also on-going, 1877, from on + going (see go).
onion (n.): early 12c., from Anglo-French union, Old French oignon "onion" (formerly also oingnon), and directly from Latin unionem (nominative unio), colloquial rustic Roman for "a kind of onion," also "pearl" (via notion of a string of onions), literally "one, unity;" sense connection is the successive layers of an onion, in contrast with garlic or cloves.

Old English had ynne (in ynne-leac), from the same Latin source, which also produced Irish inniun, Welsh wynwyn and similar words in Germanic. In Dutch, the ending in -n was mistaken for a plural inflection and new singular ui formed. The usual Indo-European name is represented by Greek kromion, Irish crem, Welsh craf, Old English hramsa, Lithuanian kremuse.

The usual Latin word was cepa, a loan from an unknown language; it is the source of Old French cive, Old English cipe, and, via Late Latin diminutive cepulla, Italian cipolla, Spanish cebolla, Polish cebula. German Zwiebel also is from this source, but altered by folk etymology in Old High German (zwibolla) from words for "two" and "ball." Onion ring is attested from 1952.

Onion dome attested from 1956; onion grass from 1883; onion skin as a type of paper from 1892. Onions, the surname, is attested from mid-12c. (Ennian), from Old Welsh Enniaun, ultimately from Latin Annianus, which was associated with Welsh einion "anvil."
oniony (adj.): 1838, from onion + -y (2). Related: Onioniness.
online (adj.): in reference to computers, "directly connected to a peripheral device," 1950 (originally as on-line).
onlooker (n.): c.1600, from on + agent noun from look (v.).
only (adj.): Old English ænlic, anlic "only, unique, solitary," literally "one-like," from an "one" (see one) + -lic "-like" (see -ly (1)). Use as an adverb and conjunction developed in Middle English. Distinction of only and alone (now usually in reference to emotional states) is unusual; in many languages the same word serves for both. German also has a distinction in allein/einzig. Phrase only-begotten (mid-15c.) is biblical, translating Latin unigenitus, Greek monogenes. The Old English form was ancenned.
onnagata (n.): in Kabuki and similar drama, a man who plays female roles, 1901, from Japanese, from onna "woman" + kata "figure."
onomastic (adj.): 1716, from French onomastique (17c.), from Greek onomastikos "of or belonging to naming," from onomastos "named," verbal adjective of onomazein "to name," from onoma "name" (see name).
onomastics (n.): "scientific study of names and naming," 1936, from onomastic; also see -ics.
onomatopoeia (n.): 1570s, from Late Latin onomatopoeia, from Greek onomatopoiia "the making of a name or word" (in imitation of a sound associated with the thing being named), from onomatopoios, from onoma (genitive onomatos) "word, name" (see name (n.)) + a derivative of poiein "compose, make" (see poet). Related: Onomatopoeic; onomatopoeial.
onomatopoeic (adj.): 1860, from French onomatopoéique or else from onomatopoeia + -ic.
Onondaga: tribe in the Iroquois Confederacy, 1684, named for its principal settlement, from Onondaga onontake, literally "on the hill."