Do tenants need to sign an addendum after you buy? (Oregon)

[Ellie Narie]

What's the official way to go about this in the state of Oregon? How do I give tenants an addendum without needing them to sign it? I am buying a property and will be managing it myself, but the current owners are using a management company. The rental agreements that tenants have are month to month. It says that they must pay rent to the management company. After I close on this place, what are the legal steps that I need to do? Do I need to write an addendum to the rental agreement saying that instead of the management company, they will need to pay rents to me? 

Do I need to have the tenants sign it (and is it okay if they don't sign it)?