I'd go with Oregon Territory as well...unless you can find a contemporary nickname for the region that would allow for a timeframe reference (like calling Oklahoma "The Nations" prior to the late 1890s, for example). One good way to find these things is to check old newspapers (some of them have been microfilmed and can be requested through interlibrary loan, or you can contact a state historical society and see if they'll print/copy a few pages for you). That lets you see what the locals called their new home.