Localities can ban loaded carry. However, only a few areas have chosen to do so. For the most part it's areas surrounding Portland.
City of Dallas - 5.100 Unlawful Possession of Weapons.
City of Beaverton – 5.08.240 Unlawful Carrying of a Loaded Firearm.
City of Oregon City - 9.24.020 Carrying or discharge of weapons.
City of Bend – Parks 5.320 Violations.
City of Independence - § 9.24.1 POSSESSION OF FIREARMS.
City of Salem – 95.095 LOADED FIRARMS.
City of Tigard - 7.32.125 Carrying Loaded Firearms.
City of Portland - 14A.60.010 Possession of a Loaded Firearm in a Public Place.
There may be others, but those are the places I know of. It does make it frustrating for non-licensed open carry. Although it won't help you, licensed carry does exempt the person from these local restrictions.
As for travelling in your car, the law is written very broad. If it's in your vehicle it is supposed to be visible. In theory, a handgun in a OWB holster while in your vehicle should be considered unconcealed. Although an officer might argue this with you, depending on where you are and how easily he can see the firearm. Of course if you pulled your shirt over the holster, it could be considered concealed. If concealed, it would need to be unloaded. This usually means, no magazine and no ammo in any magazines.
Additionally, there are still other discussions that debate if the above locality regulations apply to the inside of your vehicle.
You would best be protected from the law if you were to travel with the handgun out of reach and not accessible. Say, in your trunk. Of course this presents a problem if you ran into trouble on the side of the road.
On the other hand, if you choose to have it in reach I would make every effort to make sure the firearm is visible.
You might also check this thread:
Another thing to consider would be this book:
I'm sure others will have some good information for you as well. Also, knowing the area you will be travelling to would be helpful as well. As mentioned, above you'll find more regulations in the Portland Metro area and less in the southern and eastern parts of the state.