Definition of: o, O

(ō) noun plural o's, O's, or os, Os, or oes (ōz)
1. The 15th letter of the English alphabet: from Phoenician ayin, which was a consonant, through Greek omicron and omega, and Roman O.
2. Any sound of the letter o. See ALPHABET.
—symbol
1. Math. Zero or naught: called also round O.
2. Chem. Oxygen (symbol O).
3. Anything shaped like an O; an oval or circle; a spot or spangle: Giotto's O. See appendix (ABBREVIATIONS).