We just did this down in Florence, you can enter at the end of South Jetty, its open to non-OHV vehicles part of the year, I want to say that come May - September it's closed. There is an are up on Seaside but I don't recall the restrictions. If you get an OHV permit and flag you can drive on the beach south of the South Jetty area in Florence, as well as the dunes.
I found this; There are places along the Oregon Coast where you are allowed to drive on the beach. Some of these places are restricted to certain times of the year. The information of when and where there is beach vehicle accessibility can be found on the left side of the Official Oregon State map put out by the Oregon Department of Transportation (ODOT) – Contact (503) 986-4000 for a copy of the map. Website: www.oregon.gov/ODOT
Bigger pic of our Jeep on the beach at South Jetty.