The child was killed in rural Scioto County, about 87 miles south of Columbus. The 8-year-old was playing with a shotgun he found hidden under his parents' mattress.
Sheriff's Capt. David Hall said the parents were away Monday when the shooting occurred; however, the boys' grandmother was there babysitting five youngsters.
The Columbus (Ohio) Dispatch said Wednesday the home was a mess with dog feces and exposed electrical wiring creating what Hall termed dangerous conditions.
The identities of the family were not revealed, although the children were sent by county officials to stay with relatives while the shooting was investigated.