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Roundup or any of the glyphosphates should work fine.They usually won't cause any damage to a tree unless absorbed by the leaves or inside parts (cambium,xylem,etc.)
If you are still worried about harming the tree,you can apply the concentrated herbicide "syrup" with a brush or mop to the poison oak instead of diluting and spraying.
A big oak shold be OK as long as you don't get excessive amounts of herbicide on the leaves or around the base where it could be absorbed by the roots.
Crossbow or other 2,4-D herbicides will also do the job.
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