Online Etymology Dictionary

ordination (n.): early 15c., "the act of conferring holy orders," from Old French ordinacion (12c.) or directly from Latin ordinationem (nominative ordinatio) "a setting in order, ordinance," noun of action from past participle stem of ordinare "arrange" (see ordain).
ordnance (n.): "cannon, artillery," 1540s, a clipped form of ordinance (q.v.) which was attested from late 14c. in the sense of "military materials, provisions of war;" a sense now obsolete but which led to those of "engines for discharging missiles" (early 15c.) and "branch of the military concerned with stores and materials" (late 15c.). The shorter word was established in these distinct senses by 17c. Ordnance survey (1833), official survey of Great Britain and Ireland, was undertaken by the government under the direction of the Master-General of the Ordnance (the natural choice, gunners being thoroughly trained in surveying ranges and distances).
Ordovician (adj.): geological period following the Cambrian and preceding the Silurian, 1879, coined by English geologist Charles Lapworth (1842-1920) from Latin Ordovices, name of an ancient British tribe in North Wales. The period so called because rocks from it first were studied extensively in the region around Bala in North Wales. The tribe's name is Celtic, literally "those who fight with hammers," from Celtic base *ordo "hammer" + PIE *wik- "to fight, conquer" (see victor).
ordure (n.): late 14c., from Old French ordure "filth, uncleanliness" (12c.), from ord, ort "filthy, dirty, foul," from Latin horridus "dreadful" (see horrid).
ore (n.): 12c., a merger of Old English ora "ore, unworked metal" (related to eorþe "earth," see earth; and cognate with Low German ur "iron-containing ore," Dutch oer, Old Norse aurr "gravel"); and Old English ar "brass, copper, bronze," from Proto-Germanic *ajiz- (source also of Old Norse eir "brass, copper," German ehern "brazen," Gothic aiz "bronze"), from PIE root *aus- (2) "gold" (see aureate). The two words were not fully assimilated till 17c.; what emerged has the form of ar but the meaning of ora.
ore rotundo: Latin, literally "with round mouth," from ablative of os "mouth" (see oral) + ablative of rotundus "round" (see rotund). From Horace ("Grais ingenium, Grais dedit ore rotundo Musa loqui," in "Ars Poetica").
oread (n.): 1580s, from Latin Oreas (genitive Oreadis), from Greek Oreias "mountain nymph," from oros "mountain," perhaps from PIE root *ergh- "to mount" (see orchestra).
oregano (n.): 1771, from Spanish or American Spanish oregano, from Latin origanus, origanum, from Greek oreiganon, from oros "mountain" (see oread) + ganos "brightness, ornament." The older form of the word in English was the Latin-derived origanum (mid-13c.), also origan (early 15c.). In Europe, the dried leaves of wild marjoram; in America, a different, and more pungent, shrub.
Oregon: 1765 as the name of a large river in the west of North America, probably the modern Columbia; of uncertain and disputed origin. It seems to be of Algonquian origin. From 1848 as the name of a U.S. territory (admitted as a state 1859).
Oreo (n.): derogatory word for "black person felt to have a 'white' mentality," 1968, African-American vernacular, from the snack cookies, which consist of dark chocolate wafers and white sugar cream filling (hence "brown outside, white inside"). The cookies (made by Nabisco) date from 1912; the source of the name has been forgotten.
Orestes: son of Agamemnon and Clytaemnestra, from Greek Orestes, literally "mountaineer," from oros "mountain" (see oread).
organ (n.): fusion of late Old English organe, and Old French orgene (12c.), both meaning "musical instrument," both from Latin organa, plural of organum "a musical instrument," from Greek organon "implement, tool for making or doing; musical instrument; organ of sense, organ of the body," literally "that with which one works," from PIE *werg-ano-, from root *werg- "to do" (source also of Greek ergon "work," orgia "religious performances;" Armenian gorc "work;" Avestan vareza "work, activity;" Gothic waurkjan, Old English wyrcan "to work," Old English weorc "deed, action, something done;" Old Norse yrka "work, take effect").

Applied vaguely in late Old English to musical instruments; by late 14c. the sense of the word (used in both singular and plural form) narrowed to the musical instrument now known by that name (involving pipes supplied with wind by a bellows and worked by means of keys), though Augustine (c. 400) knew this as a specific sense of Latin organa. The meaning "body part adapted to a certain function" is attested from late 14c., from a Medieval Latin sense of Latin organum. Organist is first recorded 1590s; organ-grinder is attested from 1806.
organdy (n.): "fine transparent muslin," 1829, from French organdi, defined as "sorte de Mousseline ou toile de coton" (1725), of unknown origin. Barnhart suggests it is an alteration of Organzi, from medieval form of Urgench, city in Uzbekistan that was a cotton textile center.
organelle (n.): 1909, from Modern Latin organella, a diminutive from Latin organum "instrument," in Medieval Latin "organ of the body" (see organ).
organic (adj.): 1510s, "serving as an organ or instrument," from Latin organicus, from Greek organikos "of or pertaining to an organ, serving as instruments or engines," from organon "instrument" (see organ). Sense of "from organized living beings" is first recorded 1778 (earlier this sense was in organical, mid-15c.). Meaning "free from pesticides and fertilizers" first attested 1942. Organic chemistry is attested from 1831.
organically (adv.): 1680s in reference to bodily organs; 1862 in reference to living beings; 1841 as "as part of an organized whole;" from organic. From 1971 as "without the use of pesticides, etc."
organisation (n.): chiefly British English spelling of organization. For spelling, see -ize. Related: Organisational.
organiser (n.): chiefly British English spelling of organizer (q.v.); for spelling, see -ize.
organism (n.): 1660s, "organic structure, organization," from organize + -ism. Sense of "living animal or plant" first recorded 1842. Related: Organismic.
organist (n.): 1590s, from organ + -ist, or from or influenced by Middle French organiste, from Medieval Latin organista "one who plays an organ," from Latin organum (see organ).
organization (n.): mid-15c., "act of organizing," from Middle French organisation and directly from Medieval Latin organizationem (nominative organizatio), noun of action from past participle stem of organizare, from Latin organum "instrument, organ" (see organ). Meaning "system, establishment" is from 1873. Organization man is from title of 1956 book by American sociologist William H. Whyte (1917-1999). Related: Organizational.
organize (v.): early 15c., "construct, establish," from Middle French organiser and directly from Medieval Latin organizare, from Latin organum "instrument, organ" (see organ). Related: Organized; organizing.
organized (adj.): 1590s, "furnished with organs," past participle adjective from organize (v.). Meaning "forming a whole of interdependent parts" is from 1817. Organized crime attested from 1929.
organizer (n.): 1849, agent noun from organize.
organza (n.): 1820, from French organsin (1660s), from Italian organzino, of unknown origin; perhaps from the same source as organdy.
orgasm (n.): 1680s, "sexual climax," from French orgasme or Modern Latin orgasmus, from Greek orgasmos "excitement, swelling," from organ "be in heat, become ripe for," literally "to swell, be excited," related to orge "impulse, excitement, anger," from PIE root *wrog- "to burgeon, swell with strength" (source also of Sanskrit urja "a nourishment, sap, vigor," Old Irish ferc, ferg "anger"). Also used 17c. of other violent excitements of emotion or other bodily functions.
orgasm (v.): 1973, originally and usually in reference to a woman's sexual climax, from orgasm (n.). Related: Orgasmed; orgasming.
orgasmic (adj.): 1935, from orgasm (n.). + -ic.
orgiastic (adj.): 1690s, from Greek orgiastikos "fit for orgies, exciting," from orgiastes "one who celebrates orgies," from orgiazein "to celebrate orgies," from orgia (see orgy).
orgy (n.): 1560s, orgies (plural) "secret rites in the worship of certain Greek and Roman gods," especially Dionysus, from Middle French orgies (c. 1500, from Latin orgia), and directly from Greek orgia (plural) "secret rites," especially those of Bacchus, from PIE root *werg- "to do" (see organ). The singular, orgy, was first used in English 1660s for the extended sense of "any licentious revelry." OED says of the ancient rites that they were "celebrated with extravagant dancing, singing, drinking, etc.," which gives "etc." quite a workout.
oriel (n.): "large recessed window," mid-14c., from Old French oriol "hall, vestibule; oriel," of uncertain origin, perhaps from Medieval Latin oriolum "porch, gallery" (mid-13c.), perhaps from Vulgar Latin *auraeolum, dissimilated from aulaeolum, a diminutive of Latin aulaeum "curtain." Despite much research, the sense evolution remains obscure.
Orient (n.): c. 1300, "the East" (originally usually meaning what is now called the Middle East), from Old French orient "east" (11c.), from Latin orientem (nominative oriens) "the rising sun, the east, part of the sky where the sun rises," originally "rising" (adj.), present participle of oriri "to rise" (see orchestra). The Orient Express was a train that ran from Paris to Istanbul via Vienna 1883-1961, from the start associated with espionage and intrigue.
orient (v.): c. 1727, originally "to arrange facing east," from French s'orienter "to take one's bearings," literally "to face the east" (also the source of German orientierung), from Old French orient "east," from Latin orientum (see Orient (n.)). Extended meaning "determine bearings" first attested 1842; figurative sense is from 1850. Related: Oriented; orienting.
oriental (adj.): late 14c., from Old French oriental "eastern, from the east" (12c.) and directly from Latin orientalis "of the east," from orientem (see Orient (n.)). Originally in reference to the sky, geographical sense is attested from late 15c.; oriental carpet first recorded 1868 (in C.Latin Eastlake).
Oriental (n.): "native or inhabitant of the east," 1701, from oriental (adj.).
Orientalism (n.): in reference to character, style, trait, or idiom felt to be from the Orient, 1769, from oriental + -ism. Related: Orientalist.
orientate (v.): 1849, back-formation from orientation. Related: Orientated; orientating.
orientation (n.): 1839, originally "arrangement of a building, etc., to face east or any other specified direction," noun of action from orient (v.). Sense of "action of determining one's bearings" is from 1868. Meaning "introduction to a situation" is from 1942.
oriented (adj.): "having an orientation," 1918, past participle adjective from orient (v.)
orienteering: in reference to a competitive sport, 1948, from orient (v.).
orifice (n.): early 15c., from Middle French orifice "the opening of a wound" (14c.) and directly from Latin orificium "an opening," literally "mouth-making," from os (genitive oris) "mouth" (see oral) + facere "to make, do" (see factitious). Related: Orificial.
oriflamme (n.): sacred banner of St. Denis, late 15c., from Old French orie flambe, from Latin aurea flamma "golden flame." The ancient battle standard of the kings of France, it was of red or orange-red silk, with two or three points, and was given to the kings by the abbot of St. Denis on setting out to war. Cotgrave says it was "borne at first onely in warres made against Infidells; but afterwards vsed in all other warres; and at length vtterly lost in a battell against the Flemings." It is last mentioned in an abbey inventory of 1534.
origami (n.): 1956, from Japanese origami, from ori "fold" + kami "paper."
origin (n.): c. 1400, "ancestry, race," from Old French origine "origin, race," and directly from Latin originem (nominative origo) "a rise, commencement, beginning, source; descent, lineage, birth," from stem of oriri "arise, rise, get up; become visible, appear; be born, be descended, receive life;" figuratively "come forth, take origin, proceed, start" (of rivers, rumors, etc.), from PIE root *ergh- "to mount" (see orchestra).
original (adj.): early 14c., "first in time, earliest," from Old French original "first" (13c.) and directly from Latin originalis, from originem (nominative origo) "beginning, source, birth," from oriri "to rise" (see orchestra). The first reference is in original sin "innate depravity of man's nature," supposed to be inherited from Adam in consequence of the Fall. Related: Originally.
original (n.): "original text," late 14c., from Medieval Latin originale (see original (adj.)). Of photographs, films, sound recordings, etc., from 1918.
originality (n.): 1742, from original (adj.) + -ity. Probably after French originalité (1690s).
originate (v.): 1650s, probably a back-formation of origination. In earliest reference it meant "to trace the origin of;" meaning "to bring into existence" is from 1650s; intransitive sense of "to come into existence" is from 1775. Related: Originated; originating.
origination (n.): 1640s, from Middle French origination (15c.), from Latin originationem (nominative originatio), from originem (see original (adj.)).
originator (n.): 1818, agent noun in Latin form from originate.