fifteenth letter of the alphabet, from a character that in Phoenician was called  'ain (literally "eye") and represented "a very peculiar and to us unpronounceable guttural" [Century Dictionary]. The Greeks also lacked the sound, so when they adopted the Phoenician letters they arbitrarily changed O's value to a vowel. (Thus there is no grounds for the belief that the form of the letter represents the shape of the mouth in pronouncing it.) The Greeks later added a special character for "long" O (omega), and the original became "little o" (omicron).

In Middle English and later colloquial use, o or o' can be an abbreviation of on or of, and is still literary in some words (o'clock, Jack-o'-lantern, tam-o'-shanter, cat-o'-nine-tails, will-o'-the-wisp, etc.).

O' the common prefix in Irish surnames is from Irish ó, ua (Old Irish au, ui) "descendant." 

The "connective" -o- is the usual connecting vowel in compounds taken or formed from Greek, where it often is the vowel in the stem. "[I]t is affixed, not only to terms of Greek origin, but also to those derived from Latin (Latin compounds of which would have been formed with the L. connecting or reduced thematic vowel, -i), especially when compounds are wanted with a sense that Latin composition, even if possible, would not warrant, but which would be authorized by the principles of Greek composition." [OED]

As "zero" in Arabic numerals it is attested from c. 1600, from the similarity of shape. Similarly the O blood type (1926) was originally "zero," denoting the absence of A and B agglutinogens.

As a gauge of track in model railroads, by 1905. For o as an interjection of fear, surprise, joy, etc., see oh.

The use of the colloquial or slang -o suffix in wino, ammo, combo, kiddo, the names of the Marx Brothers, etc., "is widespread in English-speaking countries but nowhere more so than in Australia" [OED].

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oaf (n.)

1620s, auf, oph (modern form from 1630s; oafish is from 1610s), "a changeling; a foolish or otherwise defective child left by the fairies in place of another carried off," from a Scandinavian source such as Norwegian alfr "silly person," in Old Norse "elf" (see elf). Hence, "a misbegotten, deformed idiot, a simpleton" (17c.). Until recently, some dictionaries still gave the plural as oaves.

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oafish (adj.)

"like an oaf, stupid, dull," 1610s, from oaf + -ish. Related: Oafishly; oafishness.

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oak (n.)

"tree or shrub of the genus Quercus," Middle English oke, from Old English ac "oak tree" and in part from cognate Old Norse eik, both from Proto-Germanic *aiks (source also of Old Saxon and Old Frisian ek, Middle Dutch eike, Dutch eik, Old High German eih, German Eiche, Swedish ek, Danish eg), a word of uncertain origin with no certain cognates outside Germanic.

The usual Indo-European base for "oak" (*deru-) has become Modern English tree (n.). In Greek and Celtic, meanwhile, words for "oak" are from the Indo-European root for "tree." All this probably reflects the importance of the oak, the monarch of the forest, to ancient Indo-Europeans. Likewise, as there were no oaks in Iceland, the Old Norse word eik came to be used by the viking settlers there for "tree" in general.

In English the word is used in Biblical translations to render Hebrew elah (probably usually "terebinth tree") and four other words. The form in Middle English was very uncertain (oc, oek, hokke, ake, eoke, aike, hock, etc.). Oak-gall "excrescence produced by an oak tree in reaction to insects," used in making ink, is by 1712.

                            Jove's own tree,
That holds the woods in awful sovereignty,
Requires a depth of lodging in the ground ;
High as his topmost boughs to heaven ascend,
So low his roots to hell's dominion tend.
[Dryden, translating Virgil]
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oaken (adj.)

"made of, or consisting of, the wood of the oak," late 14c. (12c. in surnames and place-names), oken, from oak + -en (2). Similar formation in Old Frisian eken, Dutch eiken, Old High German eichen, German eichen, Old Norse eikinn.

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oakum (n.)

"coarse, loose fiber obtained from taking apart old hemp ropes," used for caulking the seams of wooden ships, etc., early 15c., okam, okum, from Old English acumba "tow, oakum, flax fibers separated by combing," literally "what is combed out," from Proto-Germanic *us-kambon (source of Old High German achambi). The first element is cognate with Old English a- "away, out, off;" the second element is from stem of cemban "to comb," from camb "a comb;" from PIE root *gembh- "tooth, nail."

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oar (n.)

"long wooden lever for propelling a boat," Middle English or, from Old English ar, from Proto-Germanic *airo (source also of Old Norse ar, Danish aare, Swedish åra), a word of unknown origin. Apparently unrelated to the IE root that is the source of Latin remus "oar," Greek eretēs "rower," eretmos "oar," English row (v.) and rudder. As "oar-like appendage of an animal," 1580s.

A long oar, used occasionally to assist a vessel in a calm, is a sweep, and is operated by two or more men. Small oars are sculls; one rower wielding a pair, sitting midlength of the thwart. ["Knight's American Mechanical Dictionary," 1884]
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oarlock (n.)

"hole or indentation in the gunwale of a boat where an oar rests," mid-14c., or-lok, from oar + lock (n.1).

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oarsman (n.)

"one who rows with an oar," mid-15c., from genitive of oar (n.) + man (n.).

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oasis (n.)

"fertile spot in a desert, where there is a spring or well and more or less vegetation," originally in reference to the Libyan desert, 1610s, from French oasis (18c.) and directly from Late Latin oasis, from Greek oasis, probably from Hamitic (compare Coptic wahe, ouahe "oasis," properly "dwelling place," from ouih "dwell"). The same Egyptian source produced Arabic wahah. Figurative sense of "any fertile place in the midst of a waste" is by 1800.

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