I've lived in Oregon for a very long time. I always say the best camping is usually in October. It is a beautiful time to be out and about.
One thing about the coast.... recently Oregon/ Washington had two severe storms for Sept. Very unusual. Looking like the coast; it may be very wise to watch the weather forecast. However, the mountains and eastern Oregon are sweet this time of year.
Try visiting areas around Sisters, Cascade Lakes Highway or Odell Lake Lodge areas. Most camps are open till the end of October without any crowds.