With a POD in the 1840's, how can the Oregon Country form a lasting independent government? It does not necessarily have to occupy all of the territory considered "Oregon" (or Columbia) though bonus points for those who can plausibly make it so.
Have Fremont go north rather than south in his last expedition. As the Mex-american war and the Oregon conflict begin to boil into possible wars, Fremont creates UNIS bear calf republic in Oregon rather than California. Like in OTL he does this even before the war has started. Initiating an actual conflict between Britain and the US.
Britain wins the war and the US looses claim to all the Oregon territory. The idea of the independent republic still survives in the mind of the American immigrants in Oregon (who made up.most of its population) a generation later they revolt and archive independence.
It let Newfoundland and Labrodor not join Canada and it was right next door, a very quick journey away, the colonies were given a free choice.
The West Coast...very much further away. Especially if we delay rail links. Has room to develop its own unique culture before the interior is tamed