Why did the proposed annexation of Texas cause such an uproar while Orgeon did not cause similar problems?
This is a difficult question without a clear answer in most history books. I would argue that the treaty between the US and Great Britain that set the Oregon boundary did not cause as much of an uproar for two reasons:
First, the US was at war with Mexico when the treaty was up for ratification. The South could hardly kick up too much of a fuss about Oregon while the country was involved in a war that was going to (potentially) gain a lot of territory that would be part of the South.
Second, with Oregon, the US was actually not taking as much land as some people wanted. The US settled for the boundary on the 49th parallel instead of really insisting on "Fifty Four Forty or Fight." Since the US was not pushing to take still more land, the South was less inclined to fight the treaty.