o- prefix short for ortho-4 Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014O, o (o?) n., pl. O’s Os, o’s os oes. 1. the 15th letter of the English alphabet, a vowel. 2. any spoken sound represented by this letter. 3. something shaped like an O. 4. a written or printed representation of the letter O or o. O (o?) interj., n., pl. O’s. interj. 1. (used before a name in direct address, esp. in solemn or poetic language, to lend earnestness to an appeal): Hear, O Israel! 2. (used as an expression of surprise, pain, annoyance, longing, gladness, etc.) n. 3. the exclamation ?O.? [1125?75; Middle English