Yes as long as there isnt snow on the dunes. Ive been to Coos bay in December, its a bit cold but theres no problem going onto the dunes. I always camped at Horsefall campground, there is always or was beach access at the end of the road where you could drive on the beach as well (last time I was on the beach was 5 years ago, so, I dont know if its changed). If it's your first time on the dunes or in deep sand, bring a shovel and a partner. You shouldn't get stuck in the winter months as the sand is very dense this time of year. Air down your tires to at least 20 psi, lower if the sand is really dry. You don't need to use 4 wheel drive either, its all about momentum, its a great place to learn. Don't park uphill on a sand dune, park down hill, this type of logic.