Answer: It's never a good idea to put soil over the roots of a tree. The roots can suffocate. Instead of building a raised bed, why not just plant between the roots? Choose ground covers, annuals and perennials that like dry shade and both the plants and the oak will be happy. Hosta, Liriope, daylilies, foxglove, pachysandra, vinca minor, and spotted dead nettle are all good plants for under an oak tree.
Best wishes with your garden!
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