I had to reverse-engineer this from the UK tartan registry. If you create an account with them, and request a sett, they'll email up to 5 per day to you. I don't understand why a state's official tartan should be so difficult to obtain: it seems like it ought to belong to the people of that state and be easily accessed by them.
Well, here you go, people of Oregon.
I place this image in the public domain, in the hope that it will increase interest in tartans and tartan design.
Neale Pickett <neale@woozle.org>