Looks like a good job for some 41% Glyphosate, spray it high on the bushes to get them and it won't affect what grass is there as much. If you want a good kill that will help get rid of the poison oak, spray it all good. Make clearing brush much easier, the 41% Glyphosate shouldn't hurt the big trees. We used it all the time with spot spraying in pines and hardwoods. We mixed it 2oz per gallon and that will give you a slow kill. Seems to work as good as the higher application of 4oz per gallon they label for woody small trees and brush , just kills much slower.