Ohio Rev. Code § 2731.09: Pleadings - effect.

On the return day of an alternative writ of mandamus, or such further day as the court allows, the defendant may answer as in a civil action. If the writ is allowed by a single judge, said defendant may demur.

The plaintiff may demur to the answer or reply to new matter therein, and the defendant may demur to the reply, as in a civil action.

The pleadings have the same effect, must be construed, may be amended, and issues of fact made by them must be tried, and further proceedings thereon had, in the same manner as in civil actions.