Just a question..................
How large was the oak that you transplanted?
If the tree was balled & burlapped, approximately how large was you ball?
Typically, you will be looking at around 11" for every inch of diameter.........keeping in mind that this is horizontal distance. Therefore, I you have a 4" tree about 6-12" above the flare, then you should end up with approximately a 44" ball. (These are ideal conditions, but don't always work that way in real life!) The depth varies with the species, but most roots are in the top 6-12" of soil, therefore if you have 15-20" of depth you will usually be in great shape.
The reason that I asked the questions before, was if you lost too much of the root ball, the tree would have had limited absorbing roots left to sustain life and also, the tree would have been left with minimal carbohydrate reserves for regrowth of leaves!