Online Etymology Dictionary

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 (e.g. 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" (cf. Armenian avelum).
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-.
ophiomancy (n.): ancient art of divination by the movements of snakes, 1680s, from ophio- + -mancy. Related: Ophiomantic; ophiomancer.
Ophir: name of a place mentioned in Old Testament as a source for fine gold; location still unknown. Hence Ophir-gold (1610s).
Ophiuchus: constellation (representing Aesculapius), 1650s, from Latin, from Greek ophioukhos, literally "holding a serpent," from ophis "serpent" (see ophio-) + stem of ekhein "to hold, have, keep" (see scheme (n.)). The constellation is equatorial, and Milton's "Ophiuchus huge in th' Arctick Sky" ("Paradise Lost") is a rare lapse for a poet who generally knew his astronomy.
ophthalmia (n.): "inflammation of the eye, conjunctivitis," late 14c., from Medieval Latin obtalmia, Old French obtalmie, from Late Latin ophthalmia, or directly from Greek ophthalmia, from ophthalmos (see ophthalmo-) + -ia.
ophthalmic (adj.): "pertaining to the eye," early 18c., from Latin ophthalmicus, from Greek ophthalmikos "pertaining to the eye," from ophthalmos "eye" (see ophthalmo-).
ophthalmo-: before vowels ophthalm-, word-forming element meaning "eye," mostly in plural, "the eyes," from Greek ophthalmo-, comb. form of ophthalmos "eye," originally "the seeing," of uncertain origin. Perhaps from ops "eye" (see optic) + a form related to thalamos "inner room, chamber" (see thalamus), giving the whole a sense of "eye and eye socket."
ophthalmologist (n.): 1834; see ophthalmology + -ist.
ophthalmology (n.): 1842; see ophthalmo- + -logy. Related: Ophthalmological.
ophthalmoscope (n.): 1857 in English; coined 1852 by German physician and physicist Hermann Ludwig Ferdinand von Helmholtz (1821–1894) from ophthalmo- + -scope.
opiate (n.): "medicine containing opium," early 15c., from Medieval Latin opiatus, from Latin opium (see opium). Figurative sense of "anything that dulls the feelings" is from 1640s. From 1540s in English as an adjective, "made with or containing opium."
opine (v.): "express an opinion," mid-15c., from Middle French opiner (15c.) and directly from Latin opinari "have an opinion, be of opinion, suppose, conjecture, think, judge," perhaps related to optare "to desire, choose" (see option). Related: Opined; opining.
opiniated (adj.): "obstinately attached to one's opinion," 1590s, past participle adjective from opiniate (from Latin opinio), a verb where now we use opine. Also see opinion.
opinion (n.): c.1300, from Old French opinion "opinion, view, judgements founded upon probabilities" (12c.), from Latin opinionem (nominative opinio) "opinion, conjecture, fancy, belief, what one thinks; appreciation, esteem," from stem of opinari "think, judge, suppose, opine," from PIE *op- "to choose" (see option).
Where there is much desire to learn, there of necessity will be much arguing, much writing, many opinions; for opinion in good men is but knowledge in the making. [Milton, "Areopagitica"]
opinionate (v.): "to hold an opinion," c.1600, from opinion + -ate (2); now surviving mostly in past participle adjective opinionated.
opinionated (adj.): "obstinate," c.1600, past participle adjective from opinionate.
opioid (n.): 1957, from opium + -oid.
opium (n.): late 14c., from Latin opium, from Greek opion "poppy juice, poppy," diminutive of opos "vegetable juice."
Die Religion ist der Seufzer der bedrängten Kreatur, das Gemüth einer herzlosen Welt, wie sie der Geist geistloser Zustände ist. Sie ist das Opium des Volks. [Karl Marx, "Zur Kritik der Hegel'schen Rechts-Philosophie," in "Deutsch-Französische Jahrbücher," February, 1844]
The British Opium War against China lasted from 1839-42; the name is attested from 1841.
opossum (n.): 1610, from Powhatan (Algonquian) opassum, "equivalent to a proto-Algonquian term meaning 'white dog'" [Bright].
opponent (n.): 1580s, from Latin opponentem (nominative opponens), present participle of opponere "oppose, object to," literally "set against, set opposite," from ob "against" (see ob-) + ponere "to put, set, place" (see position).
opportune (adj.): c.1400, from Old French opportun and directly from Latin opportunus "fit, convenient, suitable, favorable," from the phrase ob portum veniens "coming toward a port," in reference to the wind, from ob "to, toward" (see ob-) + portus "harbor" (see port (n.1)). Related: Opportunely.
opportunism (n.): "policy of adopting actions to circumstances while holding goals unchanged," 1870, from opportune + -ism. Cf. opportunist.
opportunist (n.): 1881, from opportunism (q.v.) + -ist. A word in Italian politics, later applied in French by Rochefort to Gambetta (1876) and then generally in English to any who seek to profit from the prevailing circumstances.
opportunistic (adj.): 1889, see opportunist + -ic. Related: Opportunistically.
opportunity (n.): late 14c., from Old French opportunite (13c.) and directly from Latin opportunitatem (nominative opportunitas) "fitness, convenience, suitableness, favorable time," from opportunus (see opportune). Opportunity cost attested from 1911. Expression opportunity knocks but once (at any man's door) attested from 1898.
opposable (adj.): 1660s, "capable of being withstood," from oppose + -able. In reference to human thumbs, from 1833. Related: Opposability.
oppose (v.): late 14c., from Old French oposer "oppose, resist, rival; contradict, state opposing point of view" (12c.), from poser "to place, lay down" (see pose (v.1)), blended with Latin opponere "oppose, object to, set against" (see opponent). Related: Opposed; opposing.
opposite (adj.): late 14c., "placed on the other side of (something)," from Old French oposite "opposite, contrary" (13c.), from Latin oppositus "standing against, opposed, opposite," past participle of opponere "set against" (see opponent). Meaning "contrary in nature or character" is from 1570s. As a noun from late 14c. As a preposition from 1758. As an adverb from 1817. Related: Oppositely.
opposition (n.): late 14c., an astrological term for the situation of two heavenly bodies exactly across from one another in the heavens, from Old French oposicion (12c.) or directly from Latin oppositionem (nominative oppositio) "act of opposing, a placing against," noun of action from past participle stem of opponere "set against" (see opponent). Meaning "that which is opposite something else" is from 1540s; meaning "contrast, antagonism" first attested 1580s; sense of "political party opposed to the one in power" is from 1704. Related: Oppositional.
oppress (v.): mid-14c., from Old French opresser "oppress, afflict; torment, smother" (13c.), from Medieval Latin oppressare, frequentative of Latin opprimere "press against, press together, press down;" figuratively "crush, put down, subdue, prosecute relentlessly" (in Late Latin "to rape"), from ob "against" (see ob-) + premere "to press, push" (see press (v.1)).
It is the due [external] restraint and not the moderation of rulers that constitutes a state of liberty; as the power to oppress, though never exercised, does a state of slavery. [St. George Tucker, "View of the Constitution of the United States," 1803]
Related: Oppressed; oppressing.
oppressed (adj.): late 14c., past participle adjective from oppress.
oppression (n.): mid-14c., "cruel or unjust use of power or authority," from Old French opression (12c.), from Latin oppressionem (nominative oppressio) "a pressing down; violence, oppression," noun of action from past participle stem of opprimere (see oppress). Meaning "action of weighing on someone's mind or spirits" is from late 14c.