It depends on what kind of oak trees you have, most oaks have a LOT of roots and they can out compete any other plant you put there.
You might build a raised bed around the tree, this must be double walled. as you do not want to cover the roots near the tree. Can not remember how far out you need to come.But if you cover the roots to much the tree will die.
In our kind of heat the coleus will need a lot of water.
