Is is sunny or rainy on the Oregon Coast in September? Yes!
In case my sense of humor does not translate, you should be prepared for rain just about any time you visit the Oregon Coast, though September often offers some of the most dependably sunny weather of the year. September’s high temperatures average in the mid to high 60s f (18.5-20c). Low temperatures average in the mid 40s to low 50s (8-10.5c). Rain showers are not uncommon, though September is one of the drier months of the year averaging 2-3.5 inches (5-9cm) for the month and the chance of rain increases toward late September and October. I recently wrote a story about the unpredictable weather on the Oregon Coast and you might enjoy reading that on the Coast Explorer website.
Mid-September is a very popular time on the Oregon Coast and often, when asked, I will say it is my favorite time of year here. Although there may be fewer families traveling (with children back in school), I would suggest making advance reservations for accommodations, especially on weekends or at popular destinations.