Ohio sellers must disclose known material defects on the residential property disclosure form, and a known elevated radon result is generally treated as one. Disclosure is required; testing before listing is not.
More detail
The obligation attaches to what you know. Ohio's residential property disclosure form asks sellers to report known conditions, and a prior test showing an elevated result, or a known mitigation system, is information a buyer is entitled to. What the law does not do is require you to test before listing. A seller who has never tested has nothing to disclose on that line. That creates a real decision rather than a legal one. Testing before listing means you may create a disclosure item you did not have. Not testing means a buyer's inspector very likely creates it for you, on their timeline, during the inspection response window when you have the least leverage and the least time to get competitive quotes. Most sellers in this market who test early do it for the second reason. A system installed before listing becomes a line in the marketing rather than a concession at the table, the post-mitigation test result goes in the file, and the buyer's inspection produces no surprise. Sellers who skip it are betting the house tests low, which in a region where elevated results are common is a real bet. One thing to avoid either way: an undisclosed test result you already have. That is the fact pattern that turns a $1,500 repair into a dispute after closing.