First, let me say that I'm sorry you find yourself in this situation because the law isn't going to bring a practical solution to your problem. You're going to have to work with your roommate to find a way to either: (i) co-exist; or (ii) have your roommate find somebody to replace you and take up your obligations under the lease (sub-lease); or (iii) convince your roommate to move out and you bring in another roommate to replace him; or (iv) one of you can move out and whoever keeps the apartment can pay full rent. Any of these alternatives are likely preferable to informing the police and landlord of your roommate's drug use.
Under Ohio law, it is a violation of a tenant's duties to engage in illegal drug use on the leased premises regardless of whether that is specifically set forth in your lease or not. So, you're correct that your roommate is in violation of the lease. The problem is that you and your roommate are probably jointly and severally liable under the lease. If you are, then you are both responsible for paying rent and abiding by all of the tenants' other duties under the lease. So, your roommate's pot smoking is really putting both of you in default of the lease. If you report this to the police and to the landlord, then the landlord will have grounds for evicting both of you even if you swear that you never smoked his pot.
If you are both evicted, then you will be jointly and severally liable for the rent for the remainder of the lease, or until the landlord finds a tenant to replace you. If the landlord believes that only your roommate is involved in illegal drug use, then it may just tell him to get out, but you would probably need to find a roommate to pick up his end of the rent unless you want to pay the full amount by yourself. Unless you know your landlord very well, you probably shouldn't tell him or her that you and your roommate are in breach of the lease for illegal drug activity because you can't be sure about the consequences.
Getting the police involved is probably not the best idea unless you believe that you are somehow endangered by your roommate's conduct. First, if the police even care about your roommate's pot smoking, a police investigation could create legal issues for you somehow or at least make your life inconvenient. Second, the police may not care about your roommate's recreational drug use and do nothing. Third, your roommate will find out that you told the police that he is a criminal and that will probably make him angry with you. Fourth, if this ever went to court (eviction, criminal, or both), it would be an extreme inconvenience.
For these reasons, it would be best to resolve this by talking to your roommate and letting him know how much this bothers you and that you cannot continue living with him if he's going to smoke like this. Make a proposal about how to handle it (i.e., tell him which of the options I set forth in the first paragraph that you would prefer). Try to work it out between the two of you. Although it is a tough conversation, it is actually much easier than bringing in the police or the landlord. Good luck.