If it is illegal, it would be a matter of state law; I am not an Ohio attorney and thus cannot advise you as to the application of Ohio state law. But I can still give you the best possible advice: The bartenders who work under you should never consume alcohol while working. The owner of the bar is subject to liability if something goes wrong. Among the most serious and predictable thing that could potentially go wrong is that an inebriated bartender will lack the judgment to stop serving an obviously drunken patron, thus subjecting the owner of the bar to potentially very significant liability, especially if the drunken customer gets into an accident.
In this case, forget the law and apply good old-fashioned common sense. Impose a new rule -- no drinking on the job. You might consider offering each bartender a free drink (just one) after his or her shift ends or free drinks on days that they don't work. And don't be afraid to fire a bartender who violates that rule.
Please note that I am a New York attorney and cannot advise you as to the application of Ohio state law. For that reason, you may wish to consult with a local attorney.