Definitions of O:

• noun:   the 15th letter of the Roman alphabet
• noun:   the blood group whose red cells carry neither the A nor B antigens

  Example: "People with type O blood are universal donors"

• noun:   a nonmetallic bivalent element that is normally a colorless odorless tasteless nonflammable diatomic gas; constitutes 21 percent of the atmosphere by volume; the most abundant element in the earth's crust