Oak bark soles have the reputation of being the best leather soles, more durable, more water resistant and more absorbent than standard leather soles.
There two main tanneries producing oak bark soles: Rendenbach and Baker (no relation).
Joh. Rendenbach Jr.
This tannery, located near Trier in Germany, has been producing oak bark sole leather since 1871.
See their website: https://www.lederfabrik-rendenbach.de/1/rendenbach-tannery/