If you stick to the I-5 you are only going to pass through a few cities that have a ban on loaded open carry to non permit holders. Salem, some Portland areas. Otherwise, I would read the handheld tri-fold .PDF in this area of the forum.
Myself, I was always a bit nervous about transporting in my vehicle without a permit. However lawfully keep 'er in the open and its legal. Our vehicles are considered "public areas" here in Oregon, which is certainly a dirty thing.
Keep those don't talk to police to the police videos in mind though. Be educated, and be smart.