The Collaborative International Dictionary of English v.0.48:
O \O\ ([=o]), interj.
An exclamation used in calling or directly addressing a
person or personified object; also, as an emotional or
impassioned exclamation expressing pain, grief, surprise,
desire, fear, etc.
For ever, O Lord, thy word is settled in heaven. --Ps.
O how love I thy law ! it is my meditation all the day.
Note: O is frequently followed by an ellipsis and that, an in
expressing a wish: "O [I wish] that Ishmael might live
before thee!" --Gen. xvii. 18; or in expressions of
surprise, indignation, or regret: "O [it is sad] that
such eyes should e'er meet other object!" --Sheridan
Note: A distinction between the use of O and oh is insisted
upon by some, namely, that O should be used only in
direct address to a person or personified object, and
should never be followed by the exclamation point,
while Oh (or oh) should be used in exclamations where
no direct appeal or address to an object is made, and
may be followed by the exclamation point or not,
according to the nature or construction of the
sentence. Some insist that oh should be used only as an
interjection expressing strong feeling. The form O,
however, is, it seems, the one most commonly employed
for both uses by modern writers and correctors for the
press. "O, I am slain!" --Shak. "O what a fair and
ministering angel!" "O sweet angel !" --Longfellow.
O for a kindling touch from that pure flame!
But she is in her grave, -- and oh
The difference to me! --Wordsworth.
Oh for a lodge in some vast wilderness! --Cowper.
We should distinguish between the sign of the
vocative and the emotional interjection, writing
O for the former, and oh for the latter. --Earle.
O dear, & O dear me! [corrupted fr. F. O Dieu! or It. O
Dio! O God! O Dio mio! O my God! --Wyman.], exclamations
expressive of various emotions, but usually promoted by
surprise, consternation, grief, pain, etc.