There is no off-roading license requirement in Oregon. You do need to complete an Oregon Parks and Recreation Department-approved ATV safety course if you are 16 years-old or over and you ride a Class I or a Class III ATV. You can visit the Oregon Online ATV safety education course website or attend a hands-on training if you are under age 16. Visit page two of Oregon's ATV Laws and Rules brochure for ATV class definitions.
Check out these Grass Valley, OR ATV/OHV resources: