A statewide collection of maps is Ralph N. Preston, Historical Maps of Oregon: Overland Stage Routes, Old Military Roads, Indian Battle Grounds, Old Forts, and Old Gold Mines (Corvallis, Oregon: Western Guide Publishers, 1972; Family History Library book Q 979.5 E3h).
A useful source on county boundaries is Erma Skyles Brown, Oregon County Boundary Change Maps, 1843-1916 (Lebanon, Oregon: End of Trail Researchers, 1970; Family History Library book 979.5 E7b; film 908033 item 2; fiche 6051182).
The Family History Library has a few Oregon maps. Large collections of maps are available at the University of Oregon in Eugene and at Oregon State University in Corvallis.
Printable maps are also available from the National Atlas of the United States -- http://nationalatlas.gov/printable.html