o

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The 15th letter of the modern English alphabet.
2. n. Any of the speech sounds represented by the letter o.
3. n. The 15th in a series.
4. n. Something shaped like the letter O.
5. n. One of the four major blood groups in the ABO system. Individuals with this blood group have neither A nor B antigens on the surface of their red blood cells, and have both anti-A and anti-B antibodies in their blood serum.
6. n. A zero.

Century Dictionary and Cyclopedia

1. The fifteenth letter and fourth vowel in our alphabet. It followed N also in the Italican systems, but was separated from it in Greek and Phenician by another character, which in the latter had the value of a sibilant, and in the former that of the compound ks (ξ). The O-character, accordingly, was the sixteenth in the Phenician alphabet, and it represented there the 'ain. a very peculiar and to us unpronounceable guttural; the Greeks (as in the case of E: see that letter) arbitrarily changed its value to that of a vowel, corresponding in quality to our “long ō.” There is no traceable Egyptian prototype for the character; the comparison of older forms is therefore as follows:
2. It thus appears that the belief, not uncommonly held, that O represents, and is imitated from, the rounded position of the lips in its utterance, is a delusion. The historical value of the letter (as already noticed) is that of our o, in note, etc., whether of both long and short quantities, as in Latin and the earliest Greek, or of short only, as in Greek after the addition to that alphabet of a special sign for long o (namely omega, Ω, ω). This vowel-sound, the name-sound of o, is found in English usage only with long quantity in accented syllables. There is no closely corresponding short vowel in standard English, but only in dialectal pronunciation, as in the New England utterance of certain words (much varying in number in different individuals): for example, home, whole, none. What we call “short o” (in not, on, etc.) is a sound of altogether different quality, very near to a true short ä (that is, a short utterance corresponding to the a of arm, father), but verging slightly toward the “broad” a (â) or o (ô) of laud, lord. “Short o” has a marked tendency to take on a “broader” sound, especially before r, and especially in America: hence the use, in the respellings of this work, of ô, which varies in different mouths from the full sound of â to that of ŏ. After these three values of the character, the next most common one is that of the oo-sonnd, the original and proper sound of u (represented in this work by ö), as in move, with the nearly corresponding short sound (marked u) in a few words, as wolf, woman. All these vowel-sounds partake of what is usually called a “labial” or a “rounded” character: that is to say, there is involved in their utterance a rounding and closing movement of the lips (and, it is held, of the whole mouth-cavity), in different degrees — least of all in ŏ, more and more in â, ō, u, ö; in the last, carried to its extreme, no closer rounding and approximation being possible. The labial action helps to give the vowel-sounds in question their fully distinctive character; but it can be more or less slighted without leaving them unrecognizable, and, in the generally indifferent habit of English pronunciation, is in a degree neglected, even in accented syllables, and yet more in unaccented. Our “long ō,” it should be added, regularly ends with a vanishing sound of oo (ö), as our ā with one of ē. O also has in many words the value of the “neutral” vowels of hut, hurt: for example, in son, come, love, work. O is further a member of several very common and important digraphs: thus, oo, the most marked representative of the ö-sound (in moon, rood, etc.), but also pronounced as u (book, look, etc.) and ŭ (blood, etc.); ou (in certain situations ow), oftenest representing a real diphthong (in out, sound, now, etc.), but also a variety of other sounds (as in through, could, ought, rough); oi (in certain situations oy), standing for a real diphthongal sound of which the first element is the “broad” o- or a-sound (for example, point, boy); oa (load, etc.), having the “long” o-sound; others, as eo (variously pronounced, as in people, yeoman, jeopard), oe (in foe, does, etc.), are comparatively rare.
3. As a medieval Roman numeral, 11.
4. As a symbol: In medieval musical notation, the sign of the tempus perfectum — that is, of triple rhythm. See mensurable music, under mensurable.
5. In modern musical notation, a null (which see)
6. In chem., the symbol of oxygen.
7. In logic, the symbol of the particular negative proposition. See A, 2 .
8. An abbreviation: Of old: as, in O. H. G., Old High German; O. T., Old Testament.
9. Of the Middle Latin octavius, a pint.
10. [lowercase] In a ship's log-book, of overcast.
11. Pl. o's, oes (ōz). Anything circular or approximately so, as resembling the shape of the letter o, as a spangle, the circle of a theater, the earth, etc.
12. An arithmetical cipher; zero: so called from its form.
13. A common interjection expressing surprise, pain, gladness, appeal, entreaty, invocation, lament, etc., according to the manner of utterance and the circumstances of the case.
14. n. An exclamation or lamentation.
15. n. Same as ho.
16. An abbreviated form of on. Commonly written o'.
17. Same as one.
18. Same as a, the indefinite article.
19. An abbreviated form of of, now commonly written o'. It is very common in colloquial speech, but is usually written and printed in the full form of. It is the established form of of in the phrase o'clock. See clock.
20. A prefix common in Irish surnames, equivalent to Mac-in Gaelic and Irish surnames (see Mac), meaning ‘son,’ as in O'Brien, O'Connor, O'Donnell, O'Sullivan, son of Brien, Connor, Donnell, etc.
21. The usual “connecting vowel,” properly the stem-vowel of the first element, of compound words taken or formed from the Greek, as in acr-o-lith, chrys-o-prase, mon-o-tone, prot-o-martyr, etc. This vowel -o- is often accented, becoming then, as in -o-logy, -o-graphy, etc., an apparent part of the second element. (See -ology.) So in -oid, properly -o-id, it has become apparently a part of the suffix. See -i-2.
22. An abbreviation
23. in electricity, of ohm;
24. of Ohio;
25. of only;
26. of opening of the circuit;
27. in psychology, of observer.

Wiktionary

1. interj. archaic The English vocative particle, used before a pronoun or the name of a person or persons to mark direct address.
2. interj. Alternative form of oh.
3. abbr. IRC Operator
4. abbr. Object, see SVO
5. abbr. Over
6. n. The fifteenth letter of the basic modern Latin alphabet.
7. n. close-mid back rounded vowel
8. n. The fifteenth letter of the English alphabet, called o and written in the Latin script.
9. n. The ordinal number fifteenth, derived from this letter of the English alphabet, called o and written in the Latin script.
10. n. The name of the Latin script letter O/o.
11. n. A zero (used in reading out numbers).

GNU Webster's 1913

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.
3. n. The letter O, or its sound.
4. n. Something shaped like the letter O; a circle or oval.
5. n. rare A cipher; zero.
6. adj. obsolete One.
7. 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.

WordNet 3.0

1. n. the 15th letter of the Roman alphabet
2. n. 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
3. n. the blood group whose red cells carry neither the A nor B antigens