Oregon formally joined the Union as a state in 1859, after holding a constitutional convention in 1857 and drafting a governing document modeled after those of several midwestern states. Prior to statehood, the territory was disputed by several countries.
The original Oregon Territory encompassed not just present-day Oregon but also Washington and much of British Columbia. The United States, Great Britain, Russia and Spain all initially claimed the territory. The United States based its claim to the land on the explorations of Lewis and Clark and the presence of trading posts set up by American citizen John Jacob Astor. Spain and Russia would eventually drop their claims, and Great Britain and the United States would agree to borders in 1846.
Residents of Oregon voted in 1857 to hold a constitutional convention prior to enabling legislation from Congress. Three major questions were put to the voters, who chose to approve the constitution and forbid slavery, but also overwhelmingly chose not to allow free African-Americans to live in the territory. Though Oregon had determined to be a free state, pro-slavery Democratic senators were elected in 1858, which caused some delay in Congressional authorization of statehood. In February of 1859, Oregon was finally granted statehood.