From Ohio History Central
In 1953, the Ohio legislature designated the Ohio Buckeye, Aesculus glabra, as Ohio's official state tree. The tree is called the buckeye because its nuts resemble the shape and color of a deer's eye. The buckeye is relatively common in Ohio, growing especially well along rivers and streams and in floodplains.
People commonly confuse the Ohio Buckeye tree with the Horse Chestnut, Aesculus hippocastanum. In addition, the Yellow Buckeye, Aesculus octandra, also grows in Ohio. It is closely related to the Ohio Buckeye, and it is difficult to differentiate between the two species.
The buckeye tree has also provided Ohioans with their principal nickname—the Buckeyes. The Ohio State University has adopted Brutus Buckeye as its mascot. Ohioans have referred to themselves as Buckeyes since at least the presidential election of 1840, when Ohio resident William Henry Harrison won the presidency. Harrison's supporters carved campaign souvenirs out of buckeye wood to illustrate their support for their fellow Ohioan.