Ditto...The sheriff who explained it to me some years ago, made it clear that Ohio law forbids the use of unmarked cars for stopping anyone on Ohio highways. I did not see anything in the earlier ORC posting that made exceptions for a cop in an unmarked car who decided to do otherwise. Failure to obey a police order only applies when the officer is readily identified as a police officer and thats not very easy to do in a plain unmarked car at 70 miles an hour, and waving a tin badge just don't get it done in the middle of a dark night.
As I recall, this law was passed in Ohio, and several other states, back in the late 40s, maybe early 50s, as a result of a serial killer who used a red light on his car to stop his victims...I believe the name was Carryl Chessman and his primary hunting ground was in California. He drew much national attention as "The Red Light Killer" and there was a book written about him. I may have the name spelled wrong and off a tad on the dates, but you get the idea.
That said, should one be pounced upon by an unmarked car while driving along the highway, the circumstances of what you're doing, the time of day, your surroundings, and a little common sense is in order. A flagrant violation on your part in the middle of a busy city block with dozens of witnesses, is a far cry from someone trying to stop you on a deserted country road at 2:00am when you have done nothing wrong. Again...common sense....Nobody is advocating that legitimate easily recognizable police orders be ignored.