Pronunciation: (ō-hī'ō),[key] —n. 1. a state in the NE central United States: a part of the Midwest. 10,797,419; 41,222 sq. mi. (106,765 sq. km). Cap.: Columbus. Abbr.: OH (for use with zip code), O.
2. a river formed by the confluence of the Allegheny and Monongahela rivers, flowing SW from Pittsburgh, Pa., to the Mississippi in S Illinois. 981 mi. (1580 km) long.