This site identifies about 60 Ohio round-type barns to date. Indiana probably has the most with Iowa, Wisconsin, and Illinois providing large numbers of round barns.
Numerous other type of barns can be found at the Ohio Barns web site, including: Rock City barns, artistic barns, barns with roof art and designs, religious themes, patriotic themes, plus much more. Some non barn sites are also featured such as: covered bridges, old water mills, castles, unique water towers, and an assorted group of miscellaneous and unusual structures.
They also have Old Gas Stations, Old Mills, Strange and Unusual Buildings and more.