I have many varieties of ferns under my burr oaks. Ostrich,lady,interrupted, christmas, japanese painted come to mind right off the bat. I had to clear a lot of brush and invasive sapplings, but I did not add any soil. The soil is rich and wonderful from years of composting oak leaves. My ferns all are happy and thriving. Roots do not seem to be a problem.

You should not build up your beds. Those roots of your lovely old oaks are very shallow and susceptible to damage from changes in grade. Always consider strongly at what might be below ground before making a lot of changes around the root/soil system of valuable trees.

I have dug, planted and replanted all over under my oaks. If I have chopped their roots, they don't care. I would say my oaks do not have shallow roots, however.