If it is an acknowledgment, then yes, you may notarize it, but you may not notarize an affidavit or other jurat certificate if that document has already been signed, as that requires an oath or affirmation prior to signing.
(C) A jurat certificate shall state that an oath or affirmation was administered to the signer with regard to the notarial act. (D)(1) A notary public shall not use an acknowledgment certificate with regard to a notarial act in which an oath or affirmation has been administered.
Only when the required notarial act is an acknowledgment—and if all requirements of the notarial act can be met—then yes… a notary may notarize a document that is already signed.
One of the most common mistakes that notaries make is not printing or signing their name exactly as it appears on their notary commission.
Failure to adminster the oath/affirmation when required is a serious violation of Ohio notary law, and can result in severe penalties! The words "Affidavit", "Sworn to and subscribed" or "Being duly sworn" in the notary certificate are your indication it is a jurat, and that the oath/affirmation is required.