I would just move. However, I understand that moving is not an option for some people. Medical marijuana may be used insofar as it is necessary to treat a qualifying medical condition. Based on your description, I am not sure whether he is smoking in his apartment or outside. Based on what you have said, the person may not breaking any Oregon law and your landlord doesn't have any obligation to make him do anything. However, if he is smoking in a public place as is defined by ORS 475.316, which does include hallways and common areas of apartment buildings, he may be breaking the law. He might be subject to a criminal violation.
The landlord does not have any obligation to evict people who break the law. The landlord MAY evict people who break laws. You cannot force the landlord to act. The landlord's obligation to you is to make the residence habitable. What you have described does not make the apartment uninhabitable as far as the law is concerned. The other tenant has a duty to not interfere with your enjoyment of your rented unit, but I am not aware of any cause of action to enforce that between tenants.
As far as I can tell, the landlord is not required to do anything in this situation and there is nothing you can force him to do. Moving might be your best option. You might also try mediation.
I am not your attorney and the suggestions I provide are general and not intended to be legal advice for your particular matter. If you are facing legal issues that affect your legal and financial interests, directly contact an attorney who can help you. This communication is not privileged or confidential.