o

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The 15th letter of the modern English alphabet.
• noun Any of the speech sounds represented by the letter o.
• noun The 15th in a series.
• noun Something shaped like the letter O.
• noun 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.
• noun A zero.

from The Century Dictionary.

• An abbreviated form of on. Commonly written o'.
• An abbreviated form of of, now commonly written o'.
• A common interjection expressing surprise, pain, gladness, appeal, entreaty, invocation, lament, etc., according to the manner of utterance and the circumstances of the case.
• 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.
• An abbreviation
• in electricity, of ohm;
• of Ohio;
• of only;
• of opening of the circuit;
• in psychology, of observer.
• Same as one.
• Same as a, the indefinite article.
• noun An exclamation or lamentation.
• noun Same as ho.
• 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.
• The fifteenth letter and fourth vowel in our alphabet.
• 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.
• As a medieval Roman numeral, 11.
• As a symbol: In medieval musical notation, the sign of the tempus perfectum — that is, of triple rhythm. See mensurable music, under mensurable.
• In modern musical notation, a null (which see)
• In chem., the symbol of oxygen.
• In logic, the symbol of the particular negative proposition. See A, 2 .
• An abbreviation: Of old: as, in O. H. G., Old High German; O. T., Old Testament.
• Of the Middle Latin octavius, a pint.
• [lowercase] In a ship's log-book, of overcast.
• 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.
• An arithmetical cipher; zero: so called from its form.

from the GNU version of the Collaborative International Dictionary of English.

• 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.
• 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.
• adjective obsolete One.
• 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.
• interjection exclamations expressive of various emotions, but usually promoted by surprise, consternation, grief, pain, etc.
• noun The letter O, or its sound.
• noun Something shaped like the letter O; a circle or oval.
• noun rare A cipher; zero.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The fifteenth letter of the basic modern Latin alphabet.
• noun close-mid back rounded vowel
• interjection archaic The English vocative particle, used before a pronoun or the name of a person or persons to mark direct address.
• interjection Alternative form of oh.
• abbreviation IRC Operator
• abbreviation Object, see SVO
• abbreviation Over
• noun The fifteenth letter of the English alphabet, called o and written in the Latin script.