Ohio makes UM/UIM optional, pays UIM as your limit less the other driver's coverage, and lets your policy bar all stacking
Ohio's UM and UIM rules are in R.C. 3937.18. The fault rule is R.C. 2315.33 and the deadline R.C. 2305.10.
Read how stacked and single-limit coverage differ in Ohio, check what section 3937.18 says, or see how Ohio UM and UIM work.