Is it safe to put oak leaves in a salad?
NO. It's not safe. Don't do it. Oaks produce acorns as well and the acorns are edible but the leaves are not.
The tannins found in the leaves, bark, and acorns of most Quercus spp produce poisoning through their effect on the intestinal tract and kidney. Tannins are potent, precipitators (astringents) of cellular protein. Oaks at any stage of growth are poisonous, but are particularly toxic when the leaf and flower buds are just opening in the spring. As the leaves mature they become less toxic. Ripe acorns are less toxic than when green. Ruminants frequently browse on oak without apparent problems provided they have ample access to normal forages.