THE 15th letter and the 4th vowel of the English alphabet. Phonetically it occupies a position between A and U, with both of which it is sometimes interchanged. O and U appear indeed to have had a common origin, and it has been supposed that the old Greek alphabet did not contain upsilon (v), while the Etruscan had U but not O; hence the frequent occurrence of O in Greek (in the double form of o and u) and of U in Latin. O is also interchanged with the Greek and Latin long e, equivalent to the English a (Gr., Lat. cor); with ou (Fr. tout, Lat. totus); uo (especially in Italian, as in cuore, Lat. cor); eu (Fr. lieu, Lat. locus); au (Fr. or, Lat. durum); ue (Span. bueno, Lat. bonus); ea (cleave, clove); i (write, wrote); el, pronounced as the English long i (Ger. nein, Eng. no); and short e (Lat. velle, volo; vermis, Eng. worm). In English O has four sounds: long, as in note; short, as in not; obscure, as in occur; like oo, as in move, wolf. There are also some exceptional cases in which it takes the sound of short u, as in love, some; of broad a, as in lord; and of short i, as in women.

It forms the following diphthongs: oa, sometimes equivalent to long o, as in coal, or broad a, as in broad; oe, sounded like long o in foe, oo in canoe, and long e in foetus; oi, having its proper sound as in voice; oo, long as in boon, short as in good, like long o as in floor, or like short u as in flood; ou, as in house, or like short u in double, oo in soup and could, long o in though, short o in hough, and broad a in ought; ow, sounded like ou in house or like long o (vow, low); and oy, like oi. O is employed particularly to express admiration, warning, compassion, and entreaty, and occurs as an interjection in all languages. - In inscriptions, O. stands for optimus. As a prefix to Irish names, it signifies "grandson of;" thus, O'Connor is equivalent to grandson of Connor. In freemasonry it denotes Orient; in French geography, ouest, west; in German geography, Ost, east. In Greek numeration o stood for 70 and o) for 800, and in the middle Latin 0 stood for 11, or with a dash over it, o, for 11,000.