O

	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 PhŒnician, 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, AS. bān; E. stone, AS. stān; E. broke, AS. brecan to break; E. bore, AS. beran to bear; E. dove, AS. 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.
n.	1.	The letter O, or its sound.
	2.	Something shaped like the letter O; a circle or oval.
	3.	A cipher; zero. Thou art an O without a figure. - Shak.
a.	1.	One.
interj.	1.	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. cxix. 89. O how love I thy law ! it is my meditation all the day. - Ps. 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 dear exclamations expressive of various emotions, but usually promoted by surprise, consternation, grief, pain, etc. - Wyman

Noun	1.	O - the blood group whose red cells carry neither the A nor B antigens; "people with type O blood are universal donors" Synonyms: group O, type O
	2.	O - a nonmetallic bivalent element that is normally a colorless odorless tasteless nonflammable diatomic gas; constitutes 21 percent of the atmosphere by volume; the most abundant element in the earth's crust Synonyms: atomic number 8, oxygen
	3.	O - the 15th letter of the Roman alphabet

(character)

O - ASCII code 79, The letter of the alphabet, not to be confused with 0 (zero) the digit.

air, atomic number 8, chemical element, element, gas, H2O, liquid oxygen, LOX, oxygen, ozone, water