How long do I have to make a claim against an estate in Ohio?

It looks like Ohio probate law allows you six months after the death of the person who owed you money to make a claim against his estate.  You can do this by sending a demand letter to the executor of the estate with the details of the claim, or filing a claim with the probate court.   If you are sending a demand letter, I recommend sending it registered mail so you have proof of when it was sent.  If probate has not been filed yet, you can seek to be appointed to administer the estate in addition to filing your claim.