kiddo (n.)

c. 1200, "the young of a goat," from a Scandinavian source such as Old Norse kið "young goat," from Proto-Germanic *kidjom (source also of Old High German kizzi, German kitze, Danish and Swedish kid), a word of uncertain origin.

The extended meaning "child" is recorded as slang by 1590s, established in informal usage by 1840s. Applied to skillful young thieves and pugilists at least since 1812. Kid stuff "something easy" is from 1913 (the phrase was in use about that time in reference to vaudeville acts or advertisements featuring children, and to child-oriented features in newspapers).

In clothing, "made of soft leather," as though from the skin of a kid, but commercially often of other skins. Hence kid glove "a glove made of kidskin leather" is from 1680s; sense of "characterized by wearing kid gloves," therefore "dainty, delicate" is from 1856.

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 2nd. ed. print, 1989].

1915, from wine + suffix as in bucko, kiddo.