Many maps of Oregon are available online. To find some of them use a search engine with the terms Oregon and Maps.
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; FHL 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; FHL 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.