Ohio Rev. Code Ann. § 5739.16
(A) Except as otherwise provided in this section, no assessment shall be made or issued against a vendor or consumer for any tax imposed by or pursuant to section 5739.02, 5739.021, 5739.023, 5739.026, or 5739.10 of the Revised Code more than four years after the return date for the period in which the sale or purchase was made, or more than four years after the return for such period is filed, whichever is later. A consumer who provides a fully completed exemption certificate pursuant to division (B) of section 5739.03 of the Revised Code may be assessed any tax imposed by or pursuant to section 5739.02, 5739.021, 5739.023, or 5739.026 of the Revised Code that results from denial of the claimed exemption within the later of a period otherwise allowed by this section or one year after the date the certificate was provided. This division does not bar an assessment: