Ohio Rev. Code § 4113.02: Contract provisions void.

Every undertaking or promise, whether written or oral, express or implied, constituting, or contained in, any contract or agreement of hiring or employment between any individual, firm, association, or corporation, and any employee or prospective employee of the same, whereby either party to such contract or agreement undertakes or promises not to join, become, or remain a member of any labor organization or of any organization of employers, or either party to such contract or agreement undertakes or promises that he will withdraw from the employment relation in the event that he joins, becomes, or remains a member of any labor organization or of any organization of employers, is contrary to public policy and void.