O, 15th letter of the alphabet. It is a usual symbol for a mid-back, rounded vowel, rather like the first part of oi. Such a vowel was represented by omicron [Gr., = little o ], its formal and positional correspondent in the Greek alphabet. English ō is a diphthong of ŏ and w. In chemistry O is the symbol of the element oxygen. In surnames O' is derived from the Irish patronymic system; e.g., O'Neill meant "grandson of Niall," probably referring originally to Niall Glundubh, high king of Ireland, 915–19 (see name).