What is the meaning of the word "o"?

Definitions:

  1. A human blood type in the ABO system, lacking both the A and B antigens. In blood transfusion, a person with blood of this group is a potential universal donor. copy to clipboard NOUN
    • Type 'O' blood is the hardest to match. copy to clipboard
  2. Abbreviation for the chemical agent 'Oxygen'. copy to clipboard ABBREVIATION
  3. The fifteenth letter of the alphabet. copy to clipboard NOUN
  4. Used in speech to mean zero. copy to clipboard NOUN
    • The year 1705 is usually pronounced seventeen 'o' five. copy to clipboard