Some things to think about. Oak tress put out some big roots. You may not be able to dig holes to get the bushes in, and if you do, the plants will have to compete with the tree for water and nutrients.

And remember, oak trees are only shady when the leaves are on the trees. When I was looking for foundation plants, I couldn't find an evergreen that could take deep shade in the summer and full sun in the winter. Maybe you have more options in hedges.

You might think about limbing the tree to allow some sunlight to get through. That would open up your options.