combo (n.)

late 14c., combinacyoun, "act of uniting (two things) in a whole; state of being so united," from Old French combination (14c., Modern French combinaison), from Late Latin combinationem (nominative combinatio) "a joining two by two," noun of action from past participle stem of combinare "to unite, yoke together," from Latin com "with, together" (see com-) + bini "two by two," adverb from bi- "twice" (from PIE root *dwo- "two").

Sense of "a whole formed by uniting" is from 1530s; specific sense of "union or association of persons for the attainment of some common end" is from 1570s. Meaning "series of moves required to open a combination lock" is from 1880; combination lock, one requiring a certain combination of moves to open it, is from 1851. Related: Combinational.

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].