Non existent. I hear there is some stuff in Bandon but limited.
I believe you can ride a portion of the Rogue River Trail in Gold Beach.
This the best site I have found for trails for the Coast Bay Area.
http://www.coostrails.com
Just S. of Coos Bay The Winchester Trails are decent. Lots of ups and downs roots and tech. Its quite the maze. I was able to link 18 miles worth of riding there last time.
The Blue Ridge Trail System is decent as well. Very similar to Winchester. Stickier mud. Not recommended wet.
Further N. You have the Eel Lake Trail in Lakeside which is fun. 8 miles out and back.
Even further N. you have Siltcoos lake trail N. of Reedsport which is ok but short and not really worth the drive.
Finally my favorite in the area is Cape Perpetua. 30 mins N. of Reedsport just S. of Yachats. This is a climb and descend. Great climb with a sweet descent. There is also a small freeride area at the very top if you can find it.