The Constitution Prevents that, or a least makes it much harder.
Article IV, Section 3: New states may be admitted by the Congress into this union; but no new states shall be formed or erected within the jurisdiction of any other state; nor any state be formed by the junction of two or more states, or parts of states, without the consent of the legislatures of the states concerned as well as of the Congress.
To form a new state, they would need approval from Oregon and Congress which is unlikely. However, there's nothing specifically in the Constitution that prevents parts of one state breaking off and joining an existing state.
If they wanted to form a new state, it would be easier to 1st break off and join Idaho, then after that try a form a new state (I'm sure Idaho would be more willing to let them go).
The multi-step approach works for me.
Thanks for the lesson - I've wondered, as an Idahoan. I've read some history on how these state lines were originally drawn. All politics, common sense not so much.
A great divider (maybe even into CA) would be, west of the Cascades, as far south as you want (Lefties). Inland Northwest could hash out the rest or generally keep existing lines (OR/WA, WA/ID, OR/ID). Or maybe eastern WA and OR would want to combine, not sure.
One of the problems I see with what the OR counties are doing is the sheer size and diversity of the land we're talking about. That's a lot to ask Idaho to bite off and would have an overwhelming effect on state politics (good or bad, who knows).