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
o, 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].