The Oregon Law says if you have a valid safety training course certificate from your state or country it will be recognized in Oregon. Or you can take the state of Oregon's on line course for off road safety and your legal. A third way is taking a safety briefing with a rental company and it is good for 30 days but generally you are renting their equipment.
I just took the State of Oregon's online course, there are 11 Chapters that are very short and a simple quiz at the end of each chapter at the end of the chapters is the final exam of 50 questions. You need to get 80% to pass. The chapter question are sometimes repeated in the exam and the Eleventh chapter is a review. This course is designed for young kids so it is very simple and reasonably quick to complete.