Yes, and yes (depending on the species of fish). Oak leaves will stain the water with tannins and drop the pH, just like Indian almond leaves (though maybe they're not quite as strong as IAL). So for fish that like tinted slightly acidic water, they will absolutely enjoy oak leaves. I used them in a betta breeding tank once before.
Theyre great! I boil mine so they sink faster and don't release as much tannins (if you have charcoal you wont get any anyways).
But my loaches hide under them and eat the white film that grows on them after a few days. My cories like to rest on them and under them too. My platy fry hide in them. Snails love them.
Theyre beneficial for food, shelter, and look pretty in a natural tank. Plus oak are sturdy so they last pretty long (i remove mine after a month and replace with fresh boiled leaves, before they even deteriorate).
Tbh anyone with bottom feeders, cories, loaches, what have you, they should be included in the tank. I even had a betta who loved hiding under the leaves.