Webster's Revised Unabridged Dictionary · 1913

O

O (ō)

1.
O, the fifteenth letter of the English alphabet, derives its form, value, and name from the Greek O, through the Latin. The letter came into the Greek from the Phonician, which possibly derived it ultimately from the Egyptian. Etymologically, the letter o is most closely related to a, e, and u; as in E. bone, Anglo-Saxon bān; E. stone, Anglo-Saxon stān; E. broke, Anglo-Saxon brecan to break; E. bore, Anglo-Saxon beran to bear; E. dove, Anglo-Saxon dūfe; E. toft, tuft; tone, tune; number, F. nombre.
2.
Among the ancients, O was a mark of triple time, from the notion that the ternary, or number 3, is the most perfect of numbers, and properly expressed by a circle, the most perfect figure.

O (ō) , noun

1.
The letter O, or its sound.
Mouthing out his hollow oes and aes. — Tennyson
2.
Something shaped like the letter O; a circle or oval.
This wooden O [Globe Theater] — Shakespeare
3.
A cipher; zero. [Rare]
Thou art an O without a figure. — Shakespeare

O'

[Ir. o a descendant.]

A prefix to Irish family names, which signifies grandson or descendant of, and is a character of dignity; as, O'Neil, O'Carrol.

O' (ō; unaccented o) , preposition

A shortened form of of or on.
At the turning o' the tide. — Shakespeare

O (ō) , adjective

[See One.]

One. [Obsolete] — Piers Plowman
Alle thre but o God. — Chaucer

O (ō) , interjection

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. — Bible (KJV) - Psalm cxix. 89
O how love I thy law! it is my meditation all the day. — Bible (KJV) - Psalm cxix. 97
O for a kindling touch from that pure flame! — Wordsworth
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 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!”

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.

Collocations (1)
O dear or O dear me! , exclamations expressive of various emotions, but usually promoted by surprise, consternation, grief, pain, etc.

O (ē)

A diphthong, employed in the Latin language, and thence in the English language, as the representative of the Greek diphthong oi. In many words in common use, e alone stands instead of o. Classicists prefer to write the diphthong oe separate in Latin words.

Also: OE