Once you hit summer season, with schools out, there is almost no way to simply wander in and find a site. Most Oregon parks sell out months in advance. Mid-June through Labor day is high travel time. Having done the Oregon coast many times during all times of the year, I would never count on finding a non-reserved site during the summer season.
I love the freedom to wander at will but, summer on the coast, not having reservations would give me heartburn trying to find a spot to camp. You might find yourself having to go far inland to find a spot.
I hope you enjoy a beautiful area and have no hassles.