What is the meaning of O?

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.
  • synonyms: oxygen, atomic number 8
• The 15th letter of the Roman alphabet.
• The blood group whose red cells carry neither the A nor B antigens.
  • usage: "people with type O blood are universal donors"
  • synonyms: type O, group O