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 (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 (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-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.
opera (n.) Look up opera at Dictionary.com
"a drama sung" [Klein], 1640s, from Italian opera, literally "a work, labor, composition," from Latin opera "work, effort" (Latin plural regarded as feminine singular), secondary (abstract) noun from operari "to work," from opus (genitive operis) "a work" (see opus). Defined in "Elson's Music Dictionary" as, "a form of musical composition evolved shortly before 1600, by some enthusiastic Florentine amateurs who sought to bring back the Greek plays to the modern stage."
No good opera plot can be sensible. ... People do not sing when they are feeling sensible. [W.H. Auden, 1961]
As a branch of dramatic art, it is attested from 1759. First record of opera glass "small binoculars for use at the theater" is from 1738. Soap opera is first recorded 1939, as a disparaging reference to daytime radio dramas sponsored by soap manufacturers.
operable (adj.) Look up operable at Dictionary.com
1640s, "practicable," from operate + -able, or else from Late Latin operabilis. Surgical sense, "capable of treatment by operation," recorded by 1904. Related: Operability.
operand (n.) Look up operand at Dictionary.com
1886, from Latin operandum, neuter gerundive of operari (see operation).
operant (adj.) Look up operant at Dictionary.com
"that works," early 15c., from Latin operantem (nominative operans), present participle of operari "to work" (see operation). Psychological sense of "involving behavior modification" coined 1937 by U.S. psychologist B.F. Skinner (as in operant conditioning, 1938, Skinner).
operate (v.) Look up operate at Dictionary.com
c. 1600, "to be in effect," back-formation from operation, or else from Latin operatus, past participle of operari "to work, labor, toil, take pains" (in Late Latin "to have effect, be active, cause"). Surgical sense is first attested 1799. Meaning "to work machinery" is from 1864 in American English. Related: Operated; operating. Operating system in the computer sense is from 1961.
operatic (adj.) Look up operatic at Dictionary.com
1749, from opera on model of dramatic.
operation (n.) Look up operation at Dictionary.com
late 14c., "action, performance, work," also "the performance of some science or art," from Old French operacion "operation, working, proceedings," from Latin operationem (nominative operatio) "a working, operation," from past participle stem of operari "to work, labor" (in Late Latin "to have effect, be active, cause"), from opera "work, effort," related to opus (genitive operis) "a work" (see opus). The surgical sense is first attested 1590s. Military sense of "series of movements and acts" is from 1749.
operational (adj.) Look up operational at Dictionary.com
1922, "pertaining to operation," from operation + -al (1). Meaning "in a state of functionality" is from 1944.
operationalization (n.) Look up operationalization at Dictionary.com
1966, noun of action from operationalize.
operationalize (v.) Look up operationalize at Dictionary.com
1954, from operational + -ize. Related: Operationalized; operationalizing.
operative (adj.) Look up operative at Dictionary.com
"producing the intended effect," early 15c., from Old French operatif (14c.) or directly from Late Latin operativus "creative, formative," from operat-, past participle stem of operari (see operation). Weakened sense of "significant, important" is from 1955.
operative (n.) Look up operative at Dictionary.com
"worker, operator," 1809, from operative (adj.); sense of "secret agent, spy" is first attested 1930, probably from its use by the Pinkerton Detective Agency as a title for their private detectives (1905).
operator (n.) Look up operator at Dictionary.com
1590s, "one who performs mechanical or surgical operations," agent noun from operate (v.) or from Late Latin operator. Meaning "one who carries on business shrewdly" is from 1828. Specific sense of "one who works a telephone switchboard" (1884) grew out of earlier meaning "one who works a telegraph" (1847).
operculum (n.) Look up operculum at Dictionary.com
1713, from Latin operculum "cover, lid," from operire "to cover, close" (see weir), with instrumental suffix *-tlom. Related: Opercular.
operetta (n.) Look up operetta at Dictionary.com
"light opera," 1775, from Italian operetta, diminutive of opera.
operose (adj.) Look up operose at Dictionary.com
"involving much labor," 1670s, from Latin operosus "taking great pains, laborious, active, industrious," from opus (genitive operis) "work" (see opus). Related: Operosity.
Ophelia Look up Ophelia at Dictionary.com
fem. proper name, from Greek opheleia "help, aid," from ophelein "to help, aid, assist," ophelos "advantage, help," from PIE root *obhel- "to avail" (cognates: Greek ophelos "advantage," Armenian avelum "increase, abound").
ophidian (adj.) Look up ophidian at Dictionary.com
"pertaining to snakes," 1883, from Greek ophidion, diminutive of ophis "serpent" (see ophio-). Hence, ophiolatry "serpent-worship" (1862), and the 2c. sect of the Ophitæ, who revered the serpent as the symbol of divine wisdom.
ophidiophobia (n.) Look up ophidiophobia at Dictionary.com
1914, "excessive fear of snakes or reptiles," from ophidio- apparently extracted from Modern Latin ophidia, a word coined arbitrarily (to provide an -ia form to serve as an order name in taxonomy) from Greek ophis "serpent" (see ophio-) + -phobia.
ophio- Look up ophio- at Dictionary.com
before vowels ophi-, word-forming element meaning "a snake, serpent," from Greek ophio-, comb. form of ophis "serpent, a snake," from PIE *ogwhi-.