I interpret your comment of not enough soil as meaing there are many roots that are close to the surface. Oaks in general do not like root disturbance especially if this is an older tree. I would not put down any lining and extra soil. The tree roots have established themselves at a certain soil level with a certain air and water dynamic. As soon as you add soil or fabric the dynamic changes. If you really wanto plant palnts I woudl use perennial ground covers, hostas or woodland natives where the planting would be minimal and stay as far away from the trunk as possible. I would just plant and do not till the soil first or add any soil. 3-4 inches of wood mulch can be used.
