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O, type O, group O | the blood group whose red cells carry neither the A nor B antigens | |
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O, o | the 15th letter of the Roman alphabet | |
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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 |