Both the Ducks and the Beavers are now selling single-game tickets. If you plan to make the Oregon trip this season, you should buy tickets for both games soon. Both teams draw big crowds.
Here is the website for the Ducks.
Here is the website for the Beavers.
We found both websites to be quite frustrating and difficult to use, and it took multiple trials using different browsers before we managed to actually purchase tickets. If you would rather deal with a human, the number for OSU tickets is 800-GO-BEAVS (800-462-3287), and for Oregon, 800-WEB-FOOT (800-932-3668).