2.the Arabic numeral; zero; cipher. 3.a major blood group. CompareABO system. 4.oxygen. o' (ə, ō), prep. 1.of:o'clock; will-o'-the-wisp. 2.Chiefly Dial.on. [Middle English; by shortening.] O' a prefix meaning “descendant,” in Irish family names:O'Brien; O'Connor. ...