onset (n.) Look up onset at Dictionary.com
1530s, "attack, assault," from on + set (n.); compare verbal phrase to set (something) on (someone). Weaker sense of "beginning, start" first recorded 1560s. Figurative use in reference to a calamity, disease, etc. is from 1580s.
onslaught (n.) Look up onslaught at Dictionary.com
1620s, anslaight, somehow from or on analogy of Dutch aanslag "attack," from Middle Dutch aenslach, from aen "on" (see on) + slach "blow," related to slaen "slay." Spelling influenced by obsolete (since c. 1400) English slaught (n.) "slaughter," from Old English sleaht (see slaughter (n.)). No record of its use in 18c.; apparently revived by Scott.
Ontario Look up Ontario at Dictionary.com
from Mohawk (Iroquoian) ontari:io "beautiful lake" or "great lake," from /-qtar-/ "lake, river." Related: Ontarian.
ontic (adj.) Look up ontic at Dictionary.com
1949, from onto- + -ic.
onto (prep.) Look up onto at Dictionary.com
1580s, as on to, from on + to. Appeared much later than parallel into. As a closed compound (on analogy of into), first recorded 1715.
onto- Look up onto- at Dictionary.com
word-forming element meaning "a being, individual; being, existence," from Greek onto-, from stem of on (genitive ontos) "being," neuter present participle of einai "to be" (see essence).
ontogeny (n.) Look up ontogeny at Dictionary.com
"development of an individual," 1872, from onto- + -geny. Related: Ontogenic; ontogenesis.
ontological (adj.) Look up ontological at Dictionary.com
1782, from ontology + -ical. Related: Ontologically
ontology (n.) Look up ontology at Dictionary.com
"metaphysical science or study of being," 1660s (Gideon Harvey), from Modern Latin ontologia (c. 1600), from onto- + -logy.
onus (n.) Look up onus at Dictionary.com
1640s, from Latin onus "load, burden," figuratively "tax, expense; trouble, difficulty," from PIE *en-es- "burden" (source of Sanskrit anah "cart, wagon"). Hence legal Latin onus probandi (1722), literally "burden of proving."
onward (adv.) Look up onward at Dictionary.com
late 14c., from on + -ward. The form onwards, with adverbial genitive -s-, is attested from c. 1600.
onwards (adv.) Look up onwards at Dictionary.com
see onward.
onymous (adj.) Look up onymous at Dictionary.com
1775, coined to provide an opposite to anonymous. Related: Onymously.
onyx (n.) Look up onyx at Dictionary.com
mid-13c., from Old French oniche "onyx" (12c.), and directly from Latin onyx (genitive onychis), from Greek onyx "onyx-stone," originally "claw, fingernail" (see nail (n.)). So called because the mineral's color sometimes resembles that of a fingernail, pink with white streaks.
oo- Look up oo- at Dictionary.com
word-forming element meaning "egg, eggs," from Greek oon "egg," cognate with Latin ovum, Old Norse egg (see egg (n.)).
oocyte (n.) Look up oocyte at Dictionary.com
1895, from oo- + -cyte (see cyto-).
oodles (n.) Look up oodles at Dictionary.com
"lots," 1869, American English (originally in a Texas context), perhaps from the caboodle in kit and caboodle (see kit).
oogenesis (n.) Look up oogenesis at Dictionary.com
"formation of the ovum," 1892, from oo- + -genesis "birth, origin, creation."
ooh Look up ooh at Dictionary.com
exclamation of pain, surprise, wonder, etc., 1916. Combined with aah from 1953. Ooh-la-la, exclamation of surprise or appreciation, is attested 1924, from French and suggestive of the supposed raciness of the French.
oolite (n.) Look up oolite at Dictionary.com
"rock consisting of fine grains of carbonate of lime," 1785, from Modern Latin oolites, from oo-, comb. form of Greek oon "egg" (cognate with Old English æg, see egg (n.)) + lithos "stone" (see litho-). So called because the rock resembles the roe of fish.
oology (n.) Look up oology at Dictionary.com
1823, from oo- + -logy. Related: Oological.
oolong (n.) Look up oolong at Dictionary.com
dark variety of Chinese tea, 1852, from Chinese wu-lung, literally "black dragon."
oom-pah Look up oom-pah at Dictionary.com
1877, imitative of bass brass instruments.
oomph Look up oomph at Dictionary.com
"sexual attractiveness," 1937, suggestive visceral physical sound. Ann Sheridan (1915–1967) was the original Hollywood oomph girl (1939).
oops Look up oops at Dictionary.com
"a natural exclamation" [OED] of surprise at doing something awkward, but attested only from 1933 (compare whoops).
Oort cloud (n.) Look up Oort cloud at Dictionary.com
in reference to the hypothetical cloud of small objects beyond Pluto that become comets, proposed 1949 by Dutch astronomer Jan Hendrick Oort (1900-1992), and named for him by 1968.
ooze (v.) Look up ooze at Dictionary.com
late 14c., wosen, verbal derivative of Old English noun wos "juice, sap," from Proto-Germanic *wosan (source of Middle Low German wose "scum"), from same source as ooze (n.). Modern spelling from late 1500s. The Old English verb was wesan. Related: Oozed; oozing.
ooze (n.) Look up ooze at Dictionary.com
"soft mud," Old English wase "soft mud, mire," from Proto-Germanic *waison (cognates: Old Saxon waso "wet ground, mire," Old Norse veisa "pond of stagnant water"), from PIE *wes- (2) "wet." Modern spelling is mid-1500s.
oozy (adj.) Look up oozy at Dictionary.com
Old English wosig "juicy, moist" (see ooze (v.)). Related: Ooziness.
op- Look up op- at Dictionary.com
assimilated form of ob- before -p-.
op-ed (adj.) Look up op-ed at Dictionary.com
1970, in reference to the page of a newspaper opposite the editorial page, usually devoted to personal opinion columns. The thing itself said to have been pioneered by the New York "World."
op. cit. Look up op. cit. at Dictionary.com
abbreviation of Latin opus citatum, literally "the work quoted."
opacity (n.) Look up opacity at Dictionary.com
1550s, "darkness of meaning, obscurity," from French opacité, from Latin opacitatem (nominative opacitas) "shade, shadiness," from opacus "shaded, dark, opaque" (see opaque). The literal sense "condition of being impervious to light" first recorded 1630s.
opafication (n.) Look up opafication at Dictionary.com
1852, from French opafication; see opacity + -fication.
opal (n.) Look up opal at Dictionary.com
1590s, from Middle French opalle (16c.), from Latin opalus (Pliny), supposedly from Greek opallios, possibly ultimately from Sanskrit upala-s "gem, precious stone." Used in Middle English in Latin form (late 14c.).
opalescence (n.) Look up opalescence at Dictionary.com
1792; see opalescent + -ence.
opalescent (adj.) Look up opalescent at Dictionary.com
1813, from opal + -escent.
opaque (adj.) Look up opaque at Dictionary.com
early 15c., opake, from Latin opacus "shaded, in the shade, shady, dark, darkened, obscure," of unknown origin. Spelling influenced after c. 1650 by French opaque (c. 1500), from the Latin. Figurative use from 1761. Related: Opaquely; opaqueness.
ope (adj.) Look up ope at Dictionary.com
short for open (adj.), early 13c. "not closed; not hidden;" originally as awake is from awaken, etc. As a verb from mid-15c. Middle English had ope-head "bare-headed" (c. 1300).
OPEC Look up OPEC at Dictionary.com
initialism (acronym) for Organization of Petroleum-Exporting Countries, founded 1960.
open (adj.) Look up open at Dictionary.com
Old English open "not closed down, raised up" (of gates, eyelids, etc.), also "exposed, evident, well-known, public," often in a bad sense, "notorious, shameless;" from Proto-Germanic *upana, literally "put or set up" (cognates: Old Norse opinn, Swedish öppen, Danish aaben, Old Saxon opan, Old Frisian epen, Old High German offan, German offen "open"), from PIE *upo "up from under, over" (cognates: Latin sub, Greek hypo; see sub-). Related to up, and throughout Germanic the word has the appearance of a past participle of *up (v.), but no such verb has been found. The source of words for "open" in many Indo-European languages seems to be an opposite of the word for "closed, shut" (such as Gothic uslukan).

Of physical spaces, "unobstructed, unencumbered," c. 1200; of rooms with unclosed entrances, c. 1300; of wounds, late 14c. Transferred sense of "frank, candid" is attested from early 14c. Of shops, etc., "available for business," it dates from 1824. Open door in reference to international trading policies is attested from 1856. Open season is first recorded 1896, of game; and figuratively 1914 of persons. Open book in the figurative sense of "person easy to understand" is from 1853. Open house "hospitality for all visitors" is first recorded 1824. Open-and-shut "simple, straightforward" first recorded 1841 in New Orleans. Open marriage, one in which the partners sleep with whomever they please, is from 1972. Open road (1817, American English) originally meant a public one; romanticized sense of "traveling as an expression of personal freedom" first recorded 1856, in Whitman.
open (n.) Look up open at Dictionary.com
early 13c., "an aperture or opening," from open (adj.). Meaning "public knowledge" (especially in out in the open) is from 1942, but compare Middle English in open (late 14c.) "manifestly, publicly." The sense of "an open competition" is from 1926, originally in a golf context.
open (v.) Look up open at Dictionary.com
Old English openian "to open, open up, disclose, reveal," also intransitive, "become manifest, be open to or exposed to," from Proto-Germanic *opanojan (cognates: Old Saxon opanon, Old Norse opna "to open," Middle Dutch, Dutch openen, Old High German offanon, German öffnen), from the source of open (adj.), but etymology suggests the adjective is older. Open up "cease to be secretive" is from 1921. Related: Opened; opening.
open-ended (adj.) Look up open-ended at Dictionary.com
1825, from open (adj.) + end (n.).
open-handed (adj.) Look up open-handed at Dictionary.com
"liberal, generous," c. 1600, from open (adj.) + -handed.
open-minded (adj.) Look up open-minded at Dictionary.com
also openminded, open minded, 1828, first recorded in Carlyle; from open (adj.) + minded. Figurative use of open (adj.) with reference to hearts, hands, etc. is from early 15c. Related: Open-mindedly; open-mindedness.
opener (n.) Look up opener at Dictionary.com
"one who opens," Old English openere, agent noun from open (v.).
opening (n.) Look up opening at Dictionary.com
Old English openung "act of opening" (a door, mouth, etc.), "disclosure, manifestation," verbal noun from present participle of open (v.). Meaning "vacant space, hole, aperture, doorway" is attested from c. 1200. Meaning "act of opening (a place, to the public)" is from late 14c. Sense of "action of beginning (something)" is from 1712; meaning "first performance of a play" is 1855; "start of an art exhibit" is from 1905. Sense of "opportunity, chance" is from 1793.
openly (adv.) Look up openly at Dictionary.com
Old English openlice "manifestly, plainly, clearly, unreservedly;" see open (adj.) + -ly (2).
openness (n.) Look up openness at Dictionary.com
Old English opennes; see open (adj.) + -ness.