On. the blood group whose red cells carry neither the A nor B antigens On. the 15th letter of the Roman alphabet On. 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 O leveln. the basic level of a subject taken in school O ringn. a gasket consisting of a flat ring of rubber or plastic; used to seal a joint against high pressure O'Brienn. Irish writer (born in 1932) O'Caseyn. Irish playwright (1880-1964) o'clockadv. according to the clock O'Connorn. United States writer (1925-1964) o'eradv. throughout a period of time O'Flahertyn. Irish writer of short stories (1896-1984) O'Haran. United States writer (1905-1970) O'Keeffen. United States painter (1887-1986) O'Neilln. United States playwright (1888-1953) O'Toolen. British actor (born in Ireland in 1932)
