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) ;
