You must have an ATV operating permit for the State of Oregon to operate your ATV on public lands in Oregon. You may use your operating permit from your home state if they also honor Oregon ATV operating permits. This reciprocity system applies to Arizona, California, Idaho, Nevada, North Dakota, Utah, and Washington. You can visit the ATV permit page for an updated list of states with reciprocity with Oregon.
Check out these Aloha, OR ATV/OHV resources:
