Online Etymology Dictionary

obviate (v.): 1590s, "to meet and do away with," from Late Latin obviatus, past participle of obviare "act contrary to, go against," from Latin obvius "that is in the way, that moves against" (see obvious). Related: Obviated; obviating.
obviation (n.): early 15c., from Medieval Latin obviationem (nominative obviatio), noun of action from past participle stem of obviare (see obviate).
obvious (adj.): 1580s, "frequently met with," from Latin obvius "that is in the way, presenting itself readily, open, exposed, commonplace," from obviam (adv.) "in the way," from ob "against" (see ob-) + viam, accusative of via "way" (see via). Meaning "plain to see, evident" is first recorded 1630s. Related: Obviously; obviousness.
oc-: assimilated form of ob- before -c-.
ocarina (n.): 1877, from Italian ocarina, diminutive of oca "goose" (so called for its shape), from Vulgar Latin *auca, from Latin avicula "small bird," diminutive of avis "bird" (see aviary).
Occam's razor: when two competing hypotheses explain the data equally well, choose the simpler. Or, as Sir William Hamilton puts it, "Neither more, nor more onerous, causes are to be assumed, than are necessary to account for the phenomena." Named for English philosopher William of Ockham or Occam (c.1285-c.1349), who expressed it with Entia non sunt multiplicanda praeter ncccssitatem.
So called after William of Occam (died about 1349): but, as a historical fact, Occam does not make much use of this principle, which belongs rather to the contemporary nominalist William Durand de St. Pourçain (died 1332). ["Century Dictionary"]
occasion (n.): late 14c., "opportunity; grounds for action, state of affairs that makes something else possible; a happening, occurrence," from Old French ochaison, ocasion "cause, reason, excuse, pretext; opportunity" (13c.) or directly from Latin occasionem (nominative occasio) "opportunity, appropriate time," in Late Latin "cause," from occasum, occasus, past participle of occidere "fall down, go down," from ob "down, away" (see ob-) + cadere "to fall" (see case (n.1)). The notion is of a "falling together," or juncture, of circumstances.
occasion (v.): mid-15c., "to bring (something) about," from occasion (n.), or else from Old French occasionner "to cause," from Medieval Latin occasionare, from Latin occasionem (see occasion (n.)). Related: Occasioned; occasioning.
occasional (adj.): late 14c., "occurring now and then," from occasion (n.) + -al (1). Meaning "casual" is 1560s. Meaning "happening on or pertaining to a particular occasion" is from 1630s. Of furniture, etc., from 1749.
occasionally (adv.): c.1400, "happening on some particular occasion," also "sometimes, happening as occasion presents itself, without regularity," from occasional + -ly (2).
Occident (n.): late 14c., "western part" (of the heavens or earth), from Old French occident (12c.) or directly from Latin occidentem (nominative occidens) "western sky, sunset, part of the sky in which the sun sets," noun use of adjective meaning "setting," from present participle of occidere "fall down, go down" (see occasion (n.)).
occidental (adj.): c.1400, from Old French occidental (14c.) and directly from Latin occidentalis "western," from occidentem (see occident). As a capitalized noun meaning "a Western person" (opposed to Oriental) from 1857.
occipital (adj.): 1540s, from Middle French occipital, from Medieval Latin occipitalis, from Latin occiput (genitive occipitis) "back of the skull," from ob "against, behind" (see ob-) + caput "head" (see capitulum).
Occitan (n.): "Old or modern Provençal; langue d'Oc," 1940, also "the northern variant of modern Provençal;" from French oc (see Languedoc).
occlude (v.): 1590s, from Latin occludere (past participle occlusus) "shut up, close up," from ob "against, up" (see ob-) + claudere "to shut, close" (see close (v.)). Of teeth, 1888 (also cf. occlusion). Related: Occluded; occluding.
occlusion (n.): 1640s, from Medieval Latin occlusionem (nominative occlusio), noun of action from past participle stem of Latin occludere (see occlude). Dentistry sense is from 1880.
occlusive (adj.): 1867, from Latin occlus-, past participle stem of occludere (see occlude) + -ive.
occult (adj.): 1530s, "secret, not divulged," from Middle French occulte and directly from Latin occultus "hidden, concealed, secret," past participle of occulere "cover over, conceal," from ob "over" (see ob-) + a verb related to celare "to hide," from PIE root *kel- "to hide" (see cell). Meaning "not apprehended by the mind, beyond the range of understanding" is from 1540s. The association with the supernatural sciences (magic, alchemy, astrology, etc.) dates from 1630s.
occultation (n.): early 15c., "disguise or concealment of identity," from Latin occultationem (nominative occultatio), noun of action from past participle stem of occultare "to hide, conceal," frequentative of occulere (see occult).
occultism (n.): 1870, from occult + -ism. Related: Occultist.
occupancy (n.): 1590s, "condition of being an occupant;" from occupant + -cy. Meaning "fact of occupying" is from 1833; that of "proportion of available space that is occupied" is attested by 1974.
occupant (n.): 1590s, "one who takes possession of something having no owner," from Middle French occupant (15c.) or directly from Latin occupantem (nominative occupans), present participle of occupare "to take possession of" (see occupy). Earlier noun form was ocupier (early 14c.).
occupation (n.): early 14c., "fact of holding or possessing;" mid-14c., "a being employed in something," also "a particular action," from Old French occupacion "pursuit, work, employment; occupancy, occupation" (12c.), from Latin occupationem (nominative occupatio) "a taking possession; business, employment," noun of action from past participle stem of occupare (see occupy). Meaning "employment, business in which one engages" is late 14c. That of "condition of being held and ruled by troops of another country" is from 1940.
occupational (adj.): 1850, from occupation + -al (1). Occupational therapy is attested by 1918; occupational risk by 1951. Related: Occupationally.
occupied (adj.): late 15c., past participle adjective from occupy (v.). Of countries overrun by others, from 1940, originally with reference to France.
occupier (n.): late 14c., agent noun from occupy.
occupy (v.): mid-14c., "to take possession of," also "to take up space or time, employ (someone)," irregularly borrowed from Old French occuper "occupy (a person or place), hold, seize" (13c.) or directly from Latin occupare "take over, seize, take into possession, possess, occupy," from ob "over" (see ob-) + intensive form of capere "to grasp, seize" (see capable). The final syllable of the English word is difficult to explain, but it is as old as the record; perhaps from a modification made in Anglo-French. During 16c.-17c. a common euphemism for "have sexual intercourse with" (sense attested from early 15c.), which caused it to fall from polite usage.
"A captaine? Gods light these villaines wil make the word as odious as the word occupy, which was an excellent good worde before it was il sorted." [Doll Tearsheet in "2 Henry IV"]
Related: Occupied; occupying.
occur (v.): 1520s, "meet, meet in argument," from Middle French occurrer "happen unexpectedly" or directly from Latin occurrere "run to meet, run against, befall, present itself," from ob "against, toward" (see ob-) + currere "to run" (see current (adj.)). Sense development is from "meet" to "present itself" to "appear" to "happen" ("present itself in the course of events"). Meaning "to come into one's mind" is from 1620s. Related: Occurred; occurring.
occurrence (n.): 1530s, from Middle French occurrence "unexpected happening" or directly from Medieval Latin occurrentia, from Latin occurentem (nominative occurens), present participle of occurrere (see occur).
ocean (n.): late 13c., from Old French occean "ocean" (12c., Modern French océan), from Latin oceanus, from Greek okeanos, the great river or sea surrounding the disk of the Earth (as opposed to the Mediterranean), of unknown origin. Personified as Oceanus, son of Uranus and Gaia and husband of Tethys. In early times, when the only known land masses were Eurasia and Africa, the ocean was an endless river that flowed around them. Until c.1650, commonly ocean sea, translating Latin mare oceanum. Application to individual bodies of water began 14c.; there are usually reckoned to be five of them, but this is arbitrary; also occasionally applied to smaller subdivisions, e.g. German Ocean "North Sea."
Oceania: "southern Pacific island and Australia, conceived as a continent," 1849, Modern Latin, from French Océanie (c.1812). Apparently coined by Danish geographer Conrad Malte-Brun (1755-1826). Earlier in English as Oceanica (1832). Oceania was the name of one of the superstates in Orwell's "Nineteen Eighty-Four." Oceanea, name of James Harrington's 17c. ideal state, later was applied to the British empire.
oceanic (adj.): 1650s, probably from French océanique, from océan (see ocean).
oceanographer (n.): 1886, agent noun from oceanography.
oceanography (n.): 1859, coined in English from ocean + -graphy; on analogy of geography. French océanographie is attested from 1580s but is said to have been rare before 1876. Related: Oceanographic.
ocelot (n.): "large wildcat of Central and South America," 1775, from French ocelot, a word formed by French naturalist Georges-Louis Leclerc, Comte de Buffon (1707-1788), from Nahuatl ocelotl "jaguar" (in full tlalocelotl, a compound formed with tlalli "field").
ocher: see ochre.
ochlocracy (n.): "government by the rabble," 1580s, from French ochlocratie (1560s), from Greek okhlokratia (Polybius) "mob rule," the lowest grade of democracy, from kratos "rule, power, strength" (see -cracy) + okhlos "mob, populace," perhaps literally "moving mass," from PIE *wogh-lo-, from root *wegh- "to go, transport" in a vehicle (see weigh). For sense development, cf. mob (n.). Related: Ochlocratic; ochlocratical. Greek also had okhlagogos "mob-leader, ochlagogue."
ochre (n.): type of clayey soil (much used in pigments), late 14c., from Old French ocre (c.1300) and directly from Late Latin ocra, from Latin ochra, from Greek ochra, from ochros "pale yellow," of unknown origin. As a color name, "brownish-yellow," it is attested from mid-15c. Related: Ochreous.
octa-: before vowels oct-, word-forming element meaning "eight," from Greek okta-, okt-, from PIE *okto(u) "eight" (see eight). The variant form octo- often appears in words taken from Latin, but the Greek form is said to be the more common in English.
octagon (n.): 1650s, from Latin octagonos, from Greek oktagononos, literally "eight-angled," from okta- comb. form of okto "eight" (see eight) + gonia "angle," related to gony "knee" (see knee). Also octogon (1650s), from French octogone.
octagonal (adj.): 1570s, from octagon + -al (1).
octahedron (n.): 1560s, from Greek oktahedron, neuter of oktahedros "eight-sided," from okta- "eight" (see octa-) + hedra "seat" (see sedentary). Related: Octahedral.
octane (n.): hydrocarbon of the methane series, 1872, coined from octo- (see octa-) + -ane; so called because it has eight carbon atoms. A fuel's octane rating, in reference to its anti-knocking quality, is attested from 1932.
octant (n.): instrument for making angular measurements in navigation or astronomy, 1731, from Late Latin octans "the eighth part," from octo "eight" (see octa-) on analogy of quadrant. In geometry, octant meant "the eighth part of a circle."
octave (n.): c.1300, utaves (plural, via Anglo-French from popular Old French form oitieve, otaves), reformed in early 15c., from Medieval Latin octava, from Latin octava dies "eighth day," fem. of octavus "eighth," from octo (see eight). Originally "period of eight days after a festival," also "eighth day after a festival" (counting both days, by inclusive reckoning, thus if the festival was on a Sunday, the octaves would be the following Sunday). Verse sense of "stanza of eight lines" is from 1580s; musical sense of "note eight diatonic degrees above (or below) a given note" is first recorded 1650s, from Latin octava (pars) "eighth part." Formerly English eighth was used in this sense (mid-15c.)
Octavia: fem. proper name, from Latin, fem. of Octavius, literally "the eighth" (see Octavian).
Octavian: masc. proper name, from Latin, from Octavius, from octavus "eighth," from octo (see eight).
But although we find so marked differences in the use of the numerals as names, it is impossible to believe that this use did not arise in the same way for all; that is, that they were at first used to distinguish children by the order of birth. But when we find them as praenomina in historical times it is evident that they no longer referred to order of birth. [George Davis Chase, "The Origin of Roman Praenomina," "Harvard Studies in Classical Philology," 1897]
octavo (n.): 1580s, printer's word for sheets folded to make eight leaves, from Latin in octavo "in the eighth," ablative of octavus "eighth" (see octave). Abbreviation is 8vo.
octo-: word-forming element, before vowels oct-, from comb. form of Latin octo "eight" (see octa-).
October: c.1050, from Latin October (mensis), from octo "eight," from PIE root *octo(u)- "eight" (see eight). Eighth month of the old Roman calendar (pre-46 B.C.E.), which began the year in March. For -ber see December. Replaced Old English winterfylleð. In Russian history, October Revolution (in which the Bolsheviks overthrew the Provisional Government) happened Nov. 7, but because Russia had not at that time adpoted the Gregorian calendar reform, this date was reckoned there (Old Style) as Oct. 25.