Online Etymology Dictionary

oology (n.): 1823, from oo- + -logy. Related: Oological.
oolong (n.): dark variety of Chinese tea, 1852, from Chinese wu-lung, literally "black dragon."
oom-pah: 1877, imitative of bass brass instruments.
oomph: "sexual attractiveness," 1937, suggestive visceral physical sound. Ann Sheridan (1915–1967) was the original Hollywood oomph girl (1939).
oops: "a natural exclamation" [OED] of surprise at doing something awkward, but attested only from 1933 (compare whoops).
Oort cloud (n.): 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.): 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.): "soft mud," Old English wase "soft mud, mire," from Proto-Germanic *waison (source also of 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.): Old English wosig "juicy, moist" (see ooze (v.)). Related: Ooziness.
op-: assimilated form of ob- before -p-.
op-ed (adj.): 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.: abbreviation of Latin opus citatum, literally "the work quoted."
opacity (n.): 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.): 1852, from French opafication; see opacity + -fication.
opal (n.): 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.): 1792; see opalescent + -ence.
opalescent (adj.): 1813, from opal + -escent.
opaque (adj.): 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.): 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: initialism (acronym) for Organization of Petroleum-Exporting Countries, founded 1960.
open (adj.): 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" (source also of 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" (source also of 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.): 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.): Old English openian "to open, open up, disclose, reveal," also intransitive, "become manifest, be open to or exposed to," from Proto-Germanic *opanojan (source also of 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.): 1825, from open (adj.) + end (n.).
open-handed (adj.): "liberal, generous," c. 1600, from open (adj.) + -handed.
open-minded (adj.): 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.): "one who opens," Old English openere, agent noun from open (v.).
opening (n.): 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.): Old English openlice "manifestly, plainly, clearly, unreservedly;" see open (adj.) + -ly (2).
openness (n.): Old English opennes; see open (adj.) + -ness.
opera (n.): "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.): 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.): 1886, from Latin operandum, neuter gerundive of operari (see operation).
operant (adj.): "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.): 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.): 1749, from opera on model of dramatic.
operation (n.): 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.): 1922, "pertaining to operation," from operation + -al (1). Meaning "in a state of functionality" is from 1944.
operationalization (n.): 1966, noun of action from operationalize.
operationalize (v.): 1954, from operational + -ize. Related: Operationalized; operationalizing.
operative (adj.): "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.): "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.): 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.): 1713, from Latin operculum "cover, lid," from operire "to cover, close" (see weir), with instrumental suffix *-tlom. Related: Opercular.
operetta (n.): "light opera," 1775, from Italian operetta, diminutive of opera.
operose (adj.): "involving much labor," 1670s, from Latin operosus "taking great pains, laborious, active, industrious," from opus (genitive operis) "work" (see opus). Related: Operosity.
Ophelia: fem. proper name, from Greek opheleia "help, aid," from ophelein "to help, aid, assist," ophelos "advantage, help," from PIE root *obhel- "to avail" (source also of Greek ophelos "advantage," Armenian avelum "increase, abound").
ophidian (adj.): "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.): 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-: before vowels ophi-, word-forming element meaning "a snake, serpent," from Greek ophio-, comb. form of ophis "serpent, a snake," from PIE *ogwhi-.