All this information from jerkies that don't ride the dunes here anymore. I went through the same thing when I moved to oregon. I wanted all my stuff legal. Well now that I've been here 2 years+ none of it is. I've been to all the different dunes prob 20 times since and had no issues. Obey the speed limits, keep from revving the shit our of your loud junk when cops are around, and don't act like a retard and you'll prob be fine. Now I've had a buddy or two that's had issues but they've been isolated. I've ridden by several cops and never been hassled.
Stickers are $10-11 depending where you get them. They just need the vin to sell it. You can write it down or take a picture. No need to take the bike in.
I wouldn't bother with oof. Unless it's a purpose drag bike. Ask that little weirdo Cam. He's got decent advice once he stops calling you a dumbass.
You will want to go online and take the stupid safety course and get your safety card. I have been asked for that out in the woods before. Just Google oregon atv safety card and you should get a buncha crap come up.