1. n. (also o) (pl. Os or O's)
1. the fifteenth letter of the alphabet.
2 (0) nought, zero (in a sequence of numerals esp. when spoken).
3 a human blood type of the ABO system. 2. abbr. (also O.) Old. 3.
symb. Chem. the element oxygen. 4. int.
1. var. of OH(1).
2 prefixed to a name in the vocative (O God).
Etymology: ME, natural excl.