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
同义词：oxygen, O, atomic number 8
the 15th letter of the Roman alphabet
同义词：O, letter o
the blood group whose red cells carry neither the A nor B antigens
