Wish I had a subsoiler run through my apple orchard before i planted it.
Mature tree can handle hardpan. Its a young tree that needs the deep soil to stay alive during dry spells. Dig as deep as you can to loosen soil. I went down about 3ft at home. Also, take a steel rod and hammer in some holes for the roots to follow.
Also, besides wayer and ants, theres vole issues too. Got them bad at home. Rabbits might leave an oak alone, but anything young and fresh is open game.