New trail being built on Mt. Hood National Forest called LaDee flat. For clubs in the area, go to pnw4wda.org, about us, clubs, Region 3. Tillamook is in the coast range west of Portland, and has trails from mild to wild. About 150 SE of Portland is Santiam Pass with a nasty trail called Quack Attack. 300 miles east is Virtue Flat over by Baker City, 5000 acres of open area and a bunch of trails some will leave you crying for the damage.
Dunes over at Sand Lake, and between Florence and Coos Bay, and another set in central Oregon out of Christmas Valley, about 100 mile se of Bend.
When usuing you rig as an Off Highway Vehicle on public land, you need to have the Oregon Class II ATV permit. Cost is $10.50 for 2 years, and a portion of the gas tax goes to fund areas for use. Find out more at Oregon Parks and Recreation, ATV button. You can get a booklet of rules, regs, and areas from them, or when you get the permit.