In Ohio, if the contract calls for a payment to be in cash, then it will be up to a court to decide if a money order is substantial compliance with that contractual requirement. You can argue that it is because there are no banks who will not honor a money order. The landlord might argue that cash is important because he doesn't want to have to take the trouble of going to the extra step of going to his bank and depositing it in his account and converting it into cash. Not sure who wins on that one.
A landlord does not have to give you a receipt. But if he won't give you a receipt, then you better have someone with you who can testify as to the fact that you gave him the cash.
