O

noun

1.
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
sourshaft atomic number 8 oxygen
2.
the 15th letter of the Roman alphabet
ethel œthel o
3.
the blood group whose red cells carry neither the A nor B antigens
group O type O