It appears that under Ohio law, whenever a security deposit exceeds one month's rent, interest must be paid on that portion of the deposit in excess of a month's rent--in this case, on $150. Since this is required by law, I do not believe the tenant may lawfully waive the interest; that is, despite the lease provision, I believe you need to pay interest. Here is a link to a relevant portion of your state law: http://codes.ohio.gov/orc/5321.16