I can only speak generally regarding the Plain fields doctrine and the drop your pants doctrine.
As far as open fields, the supreme court held that open fields to not warrant the same protections of a home as far as privacy. Generally, anytime you leave your home, you are in "public" subject to being recorded, watched, etc.. You can be on private property, but still in public view. I don't live in OR, so I am not up on any cases regarding that.
As far as the drop your pants doctrine, this is my own, not supreme court tested, but anytime you are somewhere you can drop your pants without people getting annoyed, you are generally in private.
If people get annoyed, you are in public.