it is very simple - I made "tea" aka boiling water on the bark and let it stand there to cool

If you (or anyone else) wants to go the heavy-duty, industrial-strength route, soaking pulverized oak galls (the spheroidal "oak apples" that are the tree's reaction to insect attack) will give you a generous concentration of gallic acid along with miscellaneous other tannins.

For what it is worth, gallic acid is trihydroxybenzene, a first cousin to both catechol and hydroquinone, and like them can be used as a silver halide developer.