They are mostly independent. Whenever an owner is changed, the new owner will get the owner role on the object. The exception is the unlikely case in which the new owner is the special user nobody. The nobody user never gets the owner role. Only one user can be the owner.
Many users can have the owner role.
Owner roles can be changed without changing the owner. Clearly they are similar and related. When an object is created, the creating user is the owner and is the only person who has the owner role. They are allowed to delegate ownership duties by giving others the owner role, but this is fairly rare in practice. All Zope objects may be owned.