(C) No person shall knowingly transport or have a firearm in a motor vehicle, unless the person may lawfully possess that firearm under applicable law of this state or the United States, the firearm is unloaded, and the firearm is carried in one of the following ways:
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(1) In a closed package, box, or case;
(2) In a compartment that can be reached only by leaving the vehicle;
(3) In plain sight and secured in a rack or holder made for the purpose;
(4) If the firearm is at least twenty-four inches in overall length as measured from the muzzle to the part of the stock furthest from the muzzle and if the barrel is at least eighteen inches in length, either in plain sight with the action open or the weapon stripped, or, if the firearm is of a type on which the action will not stay open or which cannot easily be stripped, in plain sight.
As noted above, you have 4 possible ways to transport a long gun in a vehicle in Ohio. The only one that stipulates the barrel be at least 18" (and the OAL at least 24") is listing (4): if you store it "in plain sight, action open" without being in a rack or holder. So for example, you could not take your unloaded 16" AR, open the action, and lay it on the back seat, in plain sight.
You could, however, store it in accordance with (1), (2), or (3). So in a closed gun case, in a secured rifle rack, or in a trunk or truck-bed, etc.