Ohio Rev. Code Ann. § 5555.21

When the proposed improvement is in two or more counties or along the county line between two or more counties in this state or extends from one county into or through one or more adjoining counties in this state, the petition described in section 5555.03 of the Revised Code may be filed with the board of county commissioners of any one of the interested counties. The board with which said petition is filed shall cause a certified copy thereof to be filed with the board of each of the other counties in which some part of the proposed improvement is situated.