Noun o has 3 senses
  1. O, type O, group O - the blood group whose red cells carry neither the A nor B antigens; "people with type O blood are universal donors"
    --1 is a kind of blood group, blood type
  2. oxygen, O, atomic number 8 - 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
    --2 is a kind of chemical element, element; gas
    --2 is a substance of water, H2O; air; ozone
    --2 has particulars: liquid oxygen, LOX
  3. O - the 15th letter of the Roman alphabet
    --3 is a kind of
    letter, letter of the alphabet, alphabetic character
    --3 is a member of Roman alphabet, Latin alphabet