Online Etymology Dictionary

oft (adv.): Old English oft "often, frequently," from Proto-Germanic *ofta- "frequently" (cf. Old Frisian ofta, Danish ofte, Old High German ofto, German oft, Old Norse opt, Gothic ufta "often"), of unknown origin. Archaic except in compounds (e.g. oft-told), and replaced by its derivative often.
often (adv.): c.1300, extended form of oft, originally before vowels and h-, probably by influence of Middle English selden "seldom." In common use from 16c., replacing oft.
oftentimes: extended form of often, attested from early 15c.
ogee (n.): "S-shaped molding," 1670s, from French ogive "diagonal rib of a vault," earlier augive, of unknown origin. Related: ogival.
ogham (n.): also ogam, ancient Irish form of writing, 1620s, from Irish ogham, from Old Irish ogam, said to be from name of its inventor, Ogma Mac Eladan. But this appears to be from Celt. *Ogmios, perhaps from PIE *og-mo- "furrow, track," thus metaphorically "incised line." This could be the source of the name of the writing style, which looks like a series of cuts or incised lines, and the inventor's name thus might be folk etymology. Related: Oghamic.
ogle (v.): 1680s, probably from Low German oeglen, frequentative of oegen "look at," from oege "eye," from Proto-Germanic *augon-, from PIE *okw- "to see" (see eye (n.)). Related to Dutch ogen "to look at," from oog "eye." Related: Ogled; ogling. The noun meaning "an amorous glance" is attested from 1711; earlier it meant "an eye" (1700).
ogre (n.): "man-eating giant," 1713, hogre (in a translation of a French version of the Arabian Nights), from French ogre, first used in Perrault's "Contes," 1697, and perhaps formed by him from Italian orco "demon, monster," from Latin Orcus "Hades," perhaps via an Italian dialect. In English, more literary than colloquial. The conjecture that it is from Byzantine Ogur "Hungarian" or some other version of that people's name (perhaps via confusion with the bloodthirsty Huns), lacks historical evidence. Related: Ogrish; ogrishness.
Ogygian (adj.): 1843, "of great antiquity or age," from Greek Ogygos, name of a mythical Attic or Boeotian king who even in classical times was thought to have lived very long ago. Also sometimes with reference to a famous flood said to have occurred in his day.
oh: 1530s, interjection expressing various emotions, a common Indo-European word (e.g. Old French ô;, oh; Latin o, oh; Greek o; Old Church Slavonic and Lithuanian o; Gothic, Dutch, German o; Old Irish a; Sanskrit a), but not found in Old English, which translated Latin oh with la or eala.
The present tendency is to restrict oh to places where it has a certain independence, & prefer o where it is proclitic or leans forward upon what follows .... [Fowler]
Often extended for emphasis, e.g. Oh, baby, stock saying from c.1918; oh, boy (1910); oh, yeah (1924). Reduplicated form oh-oh as an expression of alarm or dismay is attested from 1944. Oh-so "so very" (often sarcastic or ironic) is from 1922. Oh yeah? "really? Is that so?" attested from 1930.
Ohio: originally used of the river, from Seneca (Iroquoian) ohi:yo', proper name from ohi:yo:h, literally "good river." The Seneca also used this of the Allegheny, which they considered the headwaters of the Ohio. Related: Ohian (1819); Ohioan (1818).
ohm (n.): unit of electrical resistance, 1867, in recognition of German physicist Georg S. Ohm (1789-1854), who determined the law of the flow of electricity. Originally proposed as ohma (1861) as a unit of voltage. Related: ohmage; ohmic; ohmeter.
oho (interj.): 14c., from oh + ho.
oi: 1962, vulgar or working class pronunciation of hoy a call or shout to attract attention.
oil (n.): late 12c., "olive oil," from Anglo-French and Old North French olie, from Old French oile, uile "oil" (12c., Modern French huile), from Latin oleum "oil, olive oil" (source of Spanish, Italian olio), from Greek elaion "olive tree," from elaia (see olive). Old English æle, Dutch olie, German Öl, etc. all are from Latin. It meant "olive oil" exclusively till c.1300, when meaning began to be extended to any fatty, greasy substance. Use for "petroleum" first recorded 1520s, but not common until 19c. The artist's oils (1660s), short for oil-color (1530s), are paints made by grinding pigment in oil.
oil (v.): mid-15c., from oil (n.). Related: Oiled; oiling. An Old English verb in this sense was besmyrian.
oilcloth (n.): 1690s, "cotton or a similar fabric waterproofed with oil," from oil (n.) + cloth. In reference to an oil-treated canvas used as a cheap floor covering, 1796.
oily (adj.): 1520s, from oil (n.) + -y (2). Figurative meaning "smooth, unctuous" is from 1590s. Related: Oiliness.
oink (v.): "to make a noise like a pig," 1965, of imitative origin.
ointment (n.): late 13c., from Old French oignement "ointment, salve, unguent," from Vulgar Latin *unguimentum, from Latin unguentum (see unguent). The first -t- emerged in Old French from oint, past participle of verb oindre "to anoint."
OJ: slang abbreviation of orange juice, attested by 1963.
Ojibwa: Algonquian people of North America living along the shores of Lake Superior, 1700, from Ojibwa O'chepe'wag "plaited shoes," in reference to their puckered moccasins, which were unlike those of neighboring tribes. The older form in English is Chippewa, which is usually retained in U.S., but since c.1850 Canadian English has taken up the more phonetically correct Ojibwa, and as a result the two forms of the word have begun to be used in reference to slightly differing groups in the two countries. Some modern Chippewas prefer anishinaabe, which means "original people."
OK: 1839, only survivor of a slang fad in Boston and New York c.1838-9 for abbreviations of common phrases with deliberate, jocular misspellings (e.g. K.G. for "no go," as if spelled "know go;" N.C. for "'nuff ced;" K.Y. for "know yuse"). In the case of O.K., the abbreviation is of "oll korrect."

Probably further popularized by use as an election slogan by the O.K. Club, New York boosters of Democratic president Martin Van Buren's 1840 re-election bid, in allusion to his nickname Old Kinderhook, from his birth in the N.Y. village of Kinderhook. Van Buren lost, the word stuck, in part because it filled a need for a quick way to write an approval on a document, bill, etc. Spelled out as okeh, 1919, by Woodrow Wilson, on assumption that it represented Choctaw okeh "it is so" (a theory which lacks historical documentation); this was ousted quickly by okay after the appearance of that form in 1929. Greek immigrants to America who returned home early 20c. having picked up U.S. speech mannerisms were known in Greece as okay-boys, among other things.

The noun is first attested 1841; the verb 1888. Okey-doke is student slang first attested 1932.
okapi (n.): short-necked giraffe of central Africa, 1900, from the animal's name in Mbuba (Congo). Reported by English explorer Sir Harry Johnston (1858-1927).
okay: see OK.
oke: slang clipping of OK, attested from 1929.
Okie: "migrant agricultural worker," especially one driven from farms in Oklahoma during the Dust Bowl, 1938, short for U.S. state of Oklahoma.
"Okie use' ta mean you was from Oklahoma. Now it means you're a dirty son-of-a-bitch." [John Steinbeck, "The Grapes of Wrath," 1939]
Okinawa: largest of the Ryuku island chain, Japanese, literally "rope on the sea." Related: Okinawan.
Oklahoma: from Choctaw, literally "red people," from okla "nation, people" + homma "red." Coined by Choctaw scholar Allen Wright, later principal chief of the Choctaw Nation, and first used in the Choctaw-Chickasaw treaty of April 28, 1866. Organized as a U.S. territory 1889; admitted as a state 1907. Related: Oklahoman.
okra (n.): 1670s, from a West African language (cf. Akan nkruma "okra").
Olaf: masc. proper name, from Old Norse An-leifr, literally "ancestor's relic;" first element related to Old High German ano "ancestor;" second element related to Old English læfan "to leave" (see leave (v.)).
Olbers' paradox: "if stars are uniformly distributed through the sky, their number should counterbalance their faintness and the night sky should be as bright as the day;" named for German astronomer H.W.M. Olbers (1758-1840), who propounded it in 1826.
old (adj.): Old English ald (Anglian), eald (West Saxon) "aged, antique, primeval; elder, experienced," from West Germanic *althas "grown up, adult" (cf. Old Frisian ald, Gothic alþeis, Dutch oud, German alt), originally a past participle stem of a verb meaning "grow, nourish" (cf. Gothic alan "to grow up," Old Norse ala "to nourish"), from PIE root *al- "to grow, nourish" (cf. Greek aldaino "make grow, strengthen," althein, althainein "to get well;" Latin alere "to feed, nourish, bring up, increase," altus "high," literally "grown tall," almus "nurturing, nourishing," alumnus "fosterling, step-child;" Old Irish alim "I nourish").

The usual PIE root is *sen- (cf. senior). A few Indo-European languages distinguish words for "old" (vs. young) from words for "old" (vs. new), and some have separate words for aged persons as opposed to old things. Latin senex was used of aged living things, mostly persons, while vetus (literally "having many years") was used of inanimate things. Greek geraios was used mostly of humans; Greek palaios was used mostly of things, of persons only in a derogatory sense. Greek also had arkhaios, literally "belonging to the beginning," which parallels French ancien, used mostly with reference to things "of former times."

Old English also had fyrn "ancient," related to Old English feor "far, distant" (see far, and cf. Gothic fairneis, Old Norse forn "old, of old, of former times," Old High German firni "old, experienced"). The original Old English vowel is preserved in Scots auld, also in alderman. The original comparative and superlative (elder, eldest) are retained in particular uses.

First record of old-timer is from 1860. Expression old as the hills first recorded 1819. The good old days dates from 1828. Of old "of old times" is from late 14c. Old Glory for "the American flag" is first attested 1862. Old maid "woman who remains single well beyond the usual marrying age" is from 1520s; the card game is attested by that name from 1844. Old man "man who has lived long" is from c.1200; sense of "husband, father, boss" is from 1854, earlier (1830) it was military slang for "commanding officer;" old lady "wife, mother" is attested from c.1775. Old English is attested from 1701, originally as a type of font. Old boy originally was a former pupil of one of the English public schools. Old Testament attested from mid-14c.
old hat (adj.): "out of date," first recorded 1911. As a noun phrase, however, it had different sense previously. The "Classical Dictionary of the Vulgar Tongue" (1796) defines it as, "a woman's privities, because frequently felt."
Old World (adj.): of or pertaining to Eurasia and Africa, as opposed to the Americas, 1877.
old-fashioned (adj.): 1590s, "in an outdated style," from old + past participle of fashion (v.). As a type of cocktail, attested from 1901, American English.
Old Fashioned Tom Gin Cocktail Mix same as Holland Gin Old Fashioned Cocktail using Old Tom gin in place of Holland [George J. Kappeler, "Modern American Drinks," Akron, Ohio, 1900]
old-school (adj.): in reference to a group of people noted for conservative views or principals on some professional or political matter, 1749, from old + school (n.).
old-time (adj.): 1824, from old + time (n.). Related: Old-timey (1850).
olde: pseudo-archaic mock-antique variant of old, 1927.
olden (adj.): early 15c., from old + -en (2).
oldie (n.): "an old person," 1874; "an old tune or film," 1940, from old + -ie. Related: Oldies, which is attested by 1961 as a radio format.
oldness (n.): Old English ealdnysse; see old + -ness.
oldster (n.): 1818, colloquial, from old + -ster, on analogy of youngster.
ole: 1922, from Spanish olé "bravo!"
oleaginous (adj.): 1630s, from French oléagineux (14c.), from Latin oleaginus "of the olive," from olea "olive," alteration of oliva (see olive) by influence of oleum "oil."
oleander (n.): "rose bay," a poisonous evergreen Mediterranean shrub, c.1400, from Medieval Latin oleander, probably (by influence of Latin olea "olive tree") from Late Latin lorandrum, from Latin rhododendron (see rhododendron), itself altered by influence of Latin laurea "laurel," on resemblance of leaves. This round-about etymology is supported by the French word for it, laurier rose.
Oleg: masc. proper name; see Olga.
oleo (n.): 1884, commercial shortening of oleomargarine.
oleo-: word-forming element meaning "oil" or "oleic," from Latin oleum (see oil (n.)).
oleomargarine (n.): 1873, "butter substitute made from beef fat," from French oléomargarine (1854), from oléine (from Latin oleum "oil" + -ine, after glycerine) + margarine. It was regarded as a chemical compound of olein and margarine.
olfaction (n.): noun of action from Latin olfactus, past participle of olfacere "to smell, get the smell of" (transitive), from olere "to emit a smell" (see odor) + facere "to make" (see factitious).