Ohio's residential property disclosure form asks whether you know of previous or current radon gas on the property, the level if known, and any mitigation. It covers what you know; it does not require a test before listing.
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The form is prescribed under OAC 1301:5-6-10 and ORC 5302.30, and radon is item 4 of its hazardous materials section, next to lead-based paint, asbestos, and urea-formaldehyde insulation. The choices are yes, no, or unknown. If you hold a report, an old buyer's test, or a system in the basement, the honest answer is yes, with the level and what was done about it. The form itself tells buyers to do their own due diligence on radon and points them to EPA and Ohio Department of Health resources, so most Central Ohio buyers test anyway. That makes a pre-listing test a strategy call: learn the number on your schedule, or let the buyer's inspector learn it on theirs. For questions about your specific disclosure, ask your agent or attorney. See pre-purchase testing.