Online Etymology Dictionary

one-sided: 1833, "dealing with one side of a question or dispute," from one + side.
one-upsmanship: 1952, from noun phrase one up "scoring one more point than one's opponent" (1919).
one-way: 1906, in ref. to travel tickets; 1914 in ref. to streets; 1940 in ref. to windows, mirrors, etc.
Oneida: Iroquois tribe of upper N.Y. state, who later moved in part to Wisconsin, 1666, named for its principal settlement, from Oneida onenyote', lit. "erected stone," containing -neny- "stone" and -ot- "to stand."
oneiric: from Gk. oneiros "a dream."
oneirocritic: "a judge or interpreter of dreams," 1588 (implied in oneirocritical), from Gk. oneirokritikos "pertaining to the interpretation of dreams," from oneiros "a dream" + krites "discerner, judge" (see critic).
oneness: 1590s, from one + -ness. A re-formation of M.E. onnesse, which vanished by 13c.
onerous: c.1400, from O.Fr. (h)onereus (14c., Mod.Fr. onéreux), from L. onerosus, from onus (gen. oneris) "burden."
oneself: 1540s, one's self. Hyphenated 18c.; written as one word from c.1827, on model of himself, itself, etc.
ongoing: 1877, from on + going (see go).
onion: 1130, from Anglo-Fr. union, from O.Fr. oignon (formerly also oingnon), from L. unionem (nom. unio), colloquial rustic Roman for "a kind of onion," also "pearl," lit. "one, unity;" sense connection is the successive layers of an onion, in contrast with garlic or cloves. O.E. had ynne (in ynne-leac), from the same L. source, which also produced Ir. inniun, Welsh wynwyn and similar words in Gmc. In Du., the ending in -n was mistaken for a plural inflection and new sing. ui formed. The usual I.E. name is represented by Gk. kromion, Ir. crem, Welsh craf, O.E. hramsa, Lith. kremuse. The usual L. word was cepa, a loan from an unknown language; cf. O.Fr. cive, O.E. cipe, and, via L.L. dim. cepulla, It. cipolla, Sp. cebolla, Pol. cebula. Ger zweibel also is from this source, but altered by folk etymology in O.H.G. (zwibolla) from words for "two" and "ball." Onion ring is attested from 1952. Onions, the surname, is attested from 1159 (Ennian), from O.Wesh Enniaun, ult. from L. Annianus, which was associated with Welsh einion "anvil."
online: in ref. to computers, "directly connected to a peripheral device," is attested from 1950 (originally as on-line).
onlooker: c.1600, from on + look.
only: O.E. ænlic, anlic "only, unique, solitary," lit. "one-like," from an "one" (see one) + -lic "-ly." Distinction of only and alone (now usually in ref. to emotional states) is unusual; in many languages the same word serves for both. Ger. also has a distinction in allein/einzig. Phrase only-begotten (1450) is biblical, translating L. unigenitus, Gk. monogenes. The O.E. form was ancenned.
onnagata: in Kabuki and similar drama, a man who plays female roles, 1901, from Jap., from onna "woman" + kata "figure."
onomastic (adj.): 1716, from Fr. onomastique, from Gk. onomastikos "of or belonging to naming," from onomastos "named," verbal adj. of onomazein "to name," from onoma "name" (see name). Onomastics "scientific study of names and naming" is attested from 1936.
onomatopoeia: 1577, from L.L., from Gk. onomatopoiia "the making of a name or word" (in imitation of a sound associated with the thing being named), from onomatopoios, from onoma (gen. onomatos) "word, name" (see name) + a derivative of poiein "compose, make" (see poet).
Onondaga: tribe in the Iroquois Confederacy, 1684, named for its principal settlement, from Onondaga onontake, lit. "on the hill."
onset (n.): 1530s, "attack, assault," from on + set (n.); cf. to set (something) on (someone). Weaker sense of "beginning, start" first recorded 1560s. Figurative use in reference to a calamity, disease, etc. is from 1580s.
onslaught: 1625, anslaight, somehow from or on analogy of Du. aanslag "attack," from M.Du. aenslach, from aen "on" + slach "blow," related to slaen "slay." Spelling infl. by obs. (since c.1400) Eng. slaught (n.) "slaughter," from O.E. sleaht (see slaughter). No record of its use in 18c.; apparently revived by Scott.
Ontario: from Mohawk (Iroquoian) ontari:io "beautiful lake" or "great lake."
ontic: 1949, from Gk. ont-, comb. form of on (gen. ontos) "being," prp. of einai "to be" (see essence).
onto: 1581, as on to, from on + to. Appeared much later than parallel into. As a closed compound (on analogy of into), first recorded 1715.
ontogeny: "development of an individual," 1872, coined from Gk. on (gen. ontos) "being" (prp. of einai "to be;" see essence) + -geneia "origin," from -genes "born."
ontologically: 1846, from ontological (1782); see ontology.
ontology: "metaphysical science or study of being," 1721, from Mod.L. ontologia (coined in Fr. by Jean le Clerc, 1692), from Gk. on (gen. ontos) "being" (prp. of einai "to be;" see essence) + -logia "writing about, study of."
onus: c.1640, from L. onus (gen. oneris) "load, burden." Hence legal L. onus probandi (1722), lit. "burden of proving."
onward: late 14c., from on, on model of inward, forward. The form onwards, with adverbial genitive -s-, is attested from c.1600.
onymous: 1775, coined to provide an opposite of anonymous.
onyx: mid-13c., from O.Fr. oniche, from L. onyx (gen. onychis), from Gk. onyx "onyx-stone," originally "claw, fingernail." So called because the mineral's color sometimes resembles that of a fingernail, pink with white streaks.
oo-: comb. form meaning "egg, eggs," from Gk. oon "egg," cognate with L. ovum, O.N. egg (see egg (n.)).
oocyte: 1895, from oo- + -cyte (see cyto-).
oodles: "lots," 1869, Amer.Eng., perhaps from the caboodle in kit and caboodle (see kit).
oogenesis: formation of the ovum, 1892, from oo- + genesis.
ooh: exclamation of pain, surprise, wonder, etc., 1916. Combined with aah from 1953. Ooh-la-la, exclamation of surprise or appreciation, is attested 1924, from Fr. and suggestive of the supposed raciness of the French.
oolite: "rock consisting of fine grains of carbonate of lime," 1785, from Mod.L. oolites, from oo-, comb. form of Gk. oon "egg" (cognate with O.E. æg, see egg) + lithos "stone." So called because the rock resembles the roe of fish.
oolong: "dark variety of Chinese tea," 1852, from Chinese wu-lung, lit. "black dragon."
oom-pah: 1877, imitative of bass brass instruments.
oomph: "sexual attractiveness," 1937, suggestive of the reaction of someone hit hard in the belly. Ann Sheridan (1939) was the original Hollywood oomph girl.
oops: "a natural exclamation" [OED] of surprise at doing something awkward, but only attested from 1933.
Oort cloud: in reference to the hypothetical cloud of small objects beyond Pluto that become comets, proposed 1949 by Du. astronomer Jan Hendrick Oort (1900-1992), and named for him by 1968.
ooze (v.): late 14c., verbal derivative of O.E. noun wos "juice, sap," from P.Gmc. *wosan (cf. M.L.G. wose "scum"), from same source as ooze (n.). Modern spelling from late 1500s. The O.E. verb was wesan.
ooze (n.): "soft mud," O.E. wase "soft mud, mire," from P.Gmc. *waison (cf. O.S. waso "wet ground, mire," O.N. veisa "pond of stagnant water"), from PIE *weis- "to flow" (cf. L. virus "slime, poison," and possibly also viscum "birdlime, mistletoe"). Modern spelling is mid-1500s.
oozy: O.E. wosig (see ooze).
op-ed: 1970, page of a newspaper opposite the editorial page, usually devoted to personal opinion columns. The thing itself pioneered by the Pulitzers in the New York "World."
op. cit.: 1883, abbreviation of L. opus citatum, lit. "the work quoted."
opacity: 1560, "darkness of meaning, obscurity," from Fr. opacité, from L. opacitatem (nom. opacitas) "shade, shadiness," from opacus "shaded, dark, opaque." The lit. sense "condition of being impervious to light" first recorded 1634.
opafication: 1903, from opacity + -fication.
opal: 1590s, from Fr. opalle, from L. opalus (Pliny), supposedly from Gk. opallios, possibly ultimately from Skt. upala-s "gem, precious stone." Used in M.E. in Latin form (late 14c.).
opalescence: early 19c., from opalescent.