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].
Old English win "wine," from Proto-Germanic *winam (source also of Old Saxon, Old Frisian, Old High German win, Old Norse vin, Dutch wijn, German Wein), an early borrowing from Latin vinum "wine," from PIE *uoin-a-, related to words for "wine" in other southern European languages (Greek oinos, Albanian Ghegvênë), also Armenian (gini), Hittite (uiian(a)-), and non-Indo-European Georgian and West Semitic (Arabic wain, Hebrew yayin).
According to Watkins, probably from a lost Mediterranean language word *win-/*woin- "wine." However, Beekes argues that the word is of Indo-European origin, related to Greek itea "willow," Latin vītis "vine," and other words, and they may be derived from the root *wei- "to turn, bend."
As the wild vine was indigenous in southern Russia and in certain parts of central Europe, this assumption is acceptable from a historical point of view. However, as the cultivation of the vine started in the Mediterranean region, in the Pontus area and in the south of the Caucasus, most scholars are inclined to look for the origin of the word in these countries. This would point to non-IE origin. However, if we put the homeland of viticulture in the Pontus and the northern Balkans, the word for 'wine' might come from there. [Beekes]
Also from Latin vinum (some perhaps via Germanic) are Old Church Slavonic vino, Polish wino, Russian vino, Lithuanian vynas, Welsh gwin, Old Irish fin, Gaelic fion. Essentially the same word as vine (q.v.). Wine snob is recorded from 1951. Wine cellar is from late 14c. Wine-cooler is 1815 as "vessel in which bottled wine is kept cool;" by 1977 as a type of wine-based beverage.