1. oxygen, O, atomic number 8, chemical element, element, gas
usage: 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. O, o, letter, letter of the alphabet, alphabetic character
usage: the 15th letter of the Roman alphabet
3. O, type O, group O, blood group, blood type
usage: the blood group whose red cells carry neither the A nor B antigens; "people with type O blood are universal donors"