How did the united states acquire oregon in 1846?

The US and The British negotiated the the Anglo-American Convention of 1818 giving the US claim over Oregon because it expanded their territory.

In 1846, the Oregon boundary dispute between the U. S. and Britain was settled with the signing of the Oregon Treaty. The British gained sole possession of the land north of the 49th parallel and all of Vancouver Island, with the United States receiving the territory south of that line.