Online Etymology Dictionary

ortho-: before vowels orth-, word-forming element meaning "straight, upright, rectangular, regular; true, correct, proper," now mostly in scientific and technical compounds, from Greek ortho-, stem of orthos "straight, true, correct, regular," from PIE *eredh- "high" (cf. Sanskrit urdhvah "high, lofty, steep," Latin arduus "high, steep," Old Irish ard "high").
orthodontia (n.): 1849, from ortho- + Greek odon (genitive odontos) "tooth" (see tooth).
orthodontics (n.): 1909, from Modern Latin orthodontia + -ics.
orthodontist (n.): 1903; see orthodontia + -ist.
orthodox (adj.): mid-15c., of opinions, faith, from Late Latin orthodoxus, from Greek orthodoxos "having the right opinion," from orthos "right, true, straight" (see ortho-) + doxa "opinion, praise," from dokein "to seem," from PIE root *dek- "to take, accept" (see decent). As the name of the Eastern Church, first recorded in English 1772; in reference to a branch of Judaism, first recorded 1853.
orthodoxy (n.): 1620s, from French orthodoxie and directly from Late Latin orthodoxia, from late Greek orthodoxia "right opinion," noun of quality from orthodoxos (see orthodox).
orthogonal (adj.): 1570s, from French orthogonal, from orthogone, from Late Latin orthogonius, from Greek orthogonios "right-angled," from ortho- "straight" (see ortho-) + gonia "angle," related to gony "knee" (see knee (n.)). Related: Orthogonally.
orthographic (adj.): 1660s, from orthography + -ic. Related: Orthographically.
orthography (n.): "correct or proper spelling," mid-15c., ortographie, from Middle French orthographie (Old French ortografie, 13c.), from Latin orthographia, from Greek orthographia "correct writing," from orthos "correct" (see ortho-) + root of graphein "to write" (see -graphy). Related: Orthographer.
orthopaedics (n.): chiefly British English spelling of orthopedics; for spelling, see pedo-. Related: Orthopaedic.
orthopedic (adj.): 1840, from French orthopédique, from orthopédie, coined by French physician Nicholas Andry (1658-1742), from Greek orthos "straight, correct" (see ortho-) + paideia "rearing of children," from pais (genitive paidos) "child" (see pedo-).
orthopedics (n.): 1853, from orthopedic. Also see -ics.
orthopedist (n.): 1853, from orthopedy (1840), from French orthopédie (18c.); see orthopedic + -ist.
orthopraxy (n.): 1840, from ortho- + Greek praxis "a doing, action, performance" (see praxis).
Errata -- Page 263, line 9 from bottom, for 'orthodoxy' read orthopraxy. This is a new coin from the mint of Dr. [Andrew] Wylie [of Bloomington College, Indiana], at least I have not before noticed it. Its etymology places it in a just contrast with orthodoxy: for if that consecrated word indicates thinking right, orthopraxy will legitimately import doing right, and hence, as Mr. Wylie says, orthopraxy in the last dread day will pass the divine ordeal incomparably better than orthodoxy. O! that a zeal for orthopraxy would transcend the zeal for orthodoxy! ["The Millennial Harbinger," vol. IV, no. VIII, Bethany, Va., August 1840]
Orwellian (adj.): 1950 (first attested in Mary McCarthy), from English author George Orwell (pseudonym of Eric Blair, 1903-1950), especially in reference to his novel "Nineteen Eighty-Four." Ironically, it has come to be used in reference to the totalitarian systems he satirized.
It is as if George Orwell had conceived the nightmare instead of analyzed it, helped to create it instead of helping to dispel its euphemistic thrall. [Clive James]
oryx (n.): late 14c., from Latin oryx, from Greek oryx (genitive orygos) "North African antelope with pointed horns, the digging animal," literally "pick-axe." Used in Greek and Latin bibles to render Hebrew tho, which early English Bibles misidentified as everything from a small hibernating animal to a wild bull.
os-: frequent form of ob- before -c- and -t- in words from Latin.
Osage: name of a group of Siouxan Indians originally from Missouri, 1690s, via French, from their self-designation Wazhazhe. The ornamental tree osage orange (Toxylon pomiferum), name first attested 1817, originally was found in their country.
Oscan (adj.): of or pertaining to the ancient people of southern Italy, 1590s, from Latin Osci, Opsci (plural) "Oscans," literally "worshippers of Ops," a harvest goddess, the name related to Latin ops (genitive opis) "abundance, plenty, wealth, riches," from PIE *op- "to work, to produce in abundance" (see opus).
Oscar: masc. proper name, Old English Osgar "god's spear," from gar "spear" (see gar) + os "god" (only in personal names); see Asgard.

The statuette awarded for excellence in film acting, directing, etc., given annually since, 1928, first so called 1936. The name is said to have sprung from a 1931 remark by Margaret Herrick, secretary at Academy of Motion Picture Arts and Sciences, on seeing the statuette: "He reminds me of my Uncle Oscar." Thus the award would be named for Oscar Pierce, U.S. wheat farmer and fruit grower.
oscillate (v.): 1726, back-formation from oscillation, or else from Latin oscillatus, past participle of oscillare (see oscillation). From 1917 in electronics. Related: Oscillated; oscillating.
oscillation (n.): 1650s, from French oscillation, from Latin oscillationem (nominative oscillatio), noun of action from past participle stem of oscillare "to swing," supposed to be from oscillum "little face," literally "little mouth," a mask of open-mouthed Bacchus hung up in vineyards as a charm (the sense evolution would be via the notion of "swing in the breeze"); from PIE *os- "mouth" (see oral).
oscillator (n.): agent noun in Latin form from oscillate; 1835 of persons, 1889 in reference to electric currents.
oscilloscope (n.): "instrument for visually recording an electrical wave," 1915, a hybrid formed from Latin oscillare "to swing" (see oscillation) + -scope.
oscitant (adj.): "yawning," from Latin oscitans "listless, sluggish, lazy," present participle of oscitare "to gape, yawn," from os citare "to move the mouth" (see oral and cite).
oscitation (n.): 1540s, from Late Latin oscitationem (nominative oscitatio), noun of action from past participle stem of oscitare (see oscitant).
osculate (v.): "to kiss," 1650s, from Latin osculatus, past participle of osculari "to kiss," from osculum "a kiss; pretty mouth, sweet mouth," literally "little mouth," diminutive of os "mouth" (see oral). Related: Osculated; osculating.
osculation (n.): "kissing; a kiss," 1650s, from Latin osculationem (nominative osculatio), noun of action from past participle stem of osculari (see osculate).
osier (n.): species of willow used in basket-work, c.1300, from Old French osier "willow twig" (13c.) and directly from Medieval Latin osera "willow," ausaria "willow bed," of unknown origin, perhaps from Gaulish. Old English had the word as oser, from Medieval Latin.
Osiris: name of a principal god of Egypt, judge of the dead, from Latin Osiris, from Greek, from Egyptian Asar. Related: Osirian.
Oslo: Norwegian capital, probably based on Old Norse os "estuary, river mouth," based on the place's situation.
Osmanli: "an Ottoman Turk," 1813, from Turk. Osmanli "of or pertaining to Osman," founder of the Ottoman dynasty (he reigned 1259-1326); his name is the Turkish pronunciation of Arabic Uthman. This is the native word where English generally uses Ottoman.
osmium (n.): metallic element, 1803, coined in Modern Latin by its discoverer, English chemist Smithson Tennant (1761-1815) from Greek osme "smell, scent, odor" good or bad (cognate with Latin odor; see odor). So called for the strong smell of its oxide.
Osmond: masc. proper name, from Old English Osmund, literally "divine protection," from os "a god" (see Oscar) + -mund (see mount (n.)).
osmosis (n.): 1867, Latinized from osmose (1854), shortened from endosmosis (1830s), from endosmose "inward passage of a fluid through a porous septum" (1829), from French endo- "inward" + Greek osmos "a thrusting, a pushing," from stem of othein "to push, to thrust," from PIE *wedhe- "to push, strike" (cf. Sanskrit vadhati "pushes, strikes, destroys," Avestan vadaya- "to repulse"). Figurative sense is from 1900. Related: Osmotic (1854, from earlier endosmotic).
osprey (n.): fishing hawk, mid-15c., from Anglo-French ospriet, from Medieval Latin avis prede "bird of prey," from Latin avis praedæ, a generic term apparently confused with this specific bird in Old French on its similarity to ossifrage.
osseous (adj.): "bony," early 15c., from Medieval Latin ossous, from Latin osseus "bony, of bone," from os (genitive ossis) "bone," from PIE *ost- "bone" (cf. Sanskrit asthi, Hittite hashtai-, Greek osteon "bone," Greek ostrakon "oyster shell," Avestan ascu- "shinbone," Welsh asgwrn, Armenian oskr, Albanian asht "bone"). The word was later reformed in English (1680s), perhaps by influence of French osseux.
Ossianic (adj.): 1808, in reference to Oisin, name of a legendary Gaelic bard, literally "little fawn;" James Macpherson claimed to have collected and translated his works (1760-1763) under the name Ossian, and the poetic prose sparked a Celtic revival and fascination with the glamor of the lost world of the bards. The work turned out to be Macpherson's forgery, and the style later was regarded as bombastic, but the resulting swerve in European literature was real.
ossicle (n.): 1570s, from Latin ossiculum, diminutive of os "bone" (see osseous).
ossification (n.): 1690s, from Latin ossis "of bones," genitive of os "bone" (see osseous) + -fication.
ossifrage (n.): "sea-eagle, osprey," c.1600, from Latin ossifraga "vulture," fem. of ossifragus, literally "bone-breaker," from ossifragus (adj.) "bone-breaking," from os (genitive ossis) "bone" (see osseous) + stem of frangere "to break" (see fraction). By this name Pliny meant the lammergeier (from German, literally "lamb-vulture"), a very large Old World vulture that swallows and digests bones and was believed also to drop them from aloft to break them and get at the marrow. But in England and France, the word was transferred to the osprey, perhaps on similarity of sound between the two words.
ossify (v.): 1713, "to turn into bone," a back-formation from ossification, or else modeled on French ossifier (18c.) and formed from Latin os (genitive ossis) "bone" (see osseous) + -fy. Figurative sense is from 1858. Related: Ossified; ossifying.
ossuary (n.): "urn for the bones of the dead," 1650s, from Late Latin ossuarium "charnel house," from neuter of Latin ossuarius "of bones," from Latin os (plural ossua) "bone" (see osseous) on model of mortuarium.
ostensible (adj.): 1762, "capable of being shown, presentable," from French ostensible, from Latin ostens-, past participle stem of ostendere "to show, expose to view; to stretch out, spread before; exhibit, display," from ob "in front of" (see ob-) + tendere "to stretch" (see tenet). Meaning "apparent, professed" is from 1771.
ostensibly (adv.): 1765, from ostensible + -ly (2).
ostensive (adj.): c.1600, from Late Latin ostensivus "showing," from Latin ostensus, past participle of ostendere "to show" (see ostensible).
ostentation (n.): mid-15c., from Old French ostentacion (mid-14c.) and directly from Latin ostentationem (nominative ostentatio) "showing, exhibition, vain display," noun of action from past participle stem of ostentare "to display," frequentative of ostendere "to show" (see ostensible).
ostentatious (adj.): 1701, from ostentation + -ous. Earlier in a similar sense were ostentative (c.1600); ostentive (1590s). Related: Ostentatiously; ostentatiousness (1650s).
osteo-: before vowels oste-, word-forming element meaning "bone, bones," from Greek osteon "bone" (see osseous).
osteology (n.): 1660s, from French ostèologie, from Modern Latin osteologia, from Greek osteon "bone" (see osseous) + -logia (see -logy).