\ˈə͡ʊ], \ˈəʊ], \ˈəʊ]\
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the fifteenth letter and fourth vowel of our alphabet, its sound intermediate between a and u--with three values in English, the name-sound heard in note, the shorter sound heard in not, and the neutral vowel heard in son: as a numeral, 'nothing,' or 'zero' (formerly O=11, and ([=O])=11,000): (chem.) the symbol of oxygen: anything round or nearly so (pl. O'S, OES, pron. [=o]z).
By Thomas Davidson
By Willam Alexander Newman Dorland
By Smith Ely Jelliffe
the fifteenth letter and the fourth vowel in the English alphabet, has several different sounds ; a long sound, as in bone, tone; a short sound, as in lot, cot; a subdued sound, as in move (moov), prove (proov), and this last sound contracted, as in foot (foot), loot (loot). As a numeral, O stands for 11, and with a dash over it 11000. It is also the usual character for a cipher or nought. As an abbreviation it stands for old.