Definition of O noun from the Oxford Advanced American Dictionary

     

    O

     noun
    noun
    NAmE////
     
    o(pl. Os, O's, o's
    NAmE//oʊz//
     
    )
     
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  1. 1 [countable, uncountable] the 15th letter of the English alphabet “Orange” begins with (an) O/“O.”
  2. 2 used to mean “zero” when saying telephone numbers, etc. My number is six o three three (= 6033).
See the Oxford Advanced Learner's Dictionary entry: O