O is the fifteenth letter of the English alphabet. O is used to mean zero, for example when you are telling someone a phone number, or mentioning a year such as 1908. O is used in exclamations, especially when you are expressing strong feelings.

Doors open for seating at the earliest convenience to accommodate all seating. What is the run-time for "O"? 90 minutes without an intermission.

The Oxford Dictionary of English says that O is the archaic spelling of oh, and that it's an exclamation—that much had been clear. But for the second definition, Oxford says that O is used in the vocative, that is, as an invocation, in order to address something or someone.

(used as an expression of surprise, pain, annoyance, longing, gladness, etc.)