They do well unless the tree is competing with it for water. Other possibilities: do remember that as time passes on, your shrub gets larger and will need more water. If you have not had to increase the amount of water per watering, you may to try doing that too. And maintain 3-4" of mulch up to the drip line to prevent the moisture from evaporating rapidly in the summer. If you overwater, this can cause the oakleafs to develop root rot because they are very sensitive to wet soil. If they are wilting 24/7, that would suggest a root rot issue to me. If they wilt but recover by nightfall or by the next morning, that would be ok.

Thank you very much indeed. It does recover, so far at least, after watering. I shall increase the amount of mulch. However, I am a little perturbed by the amount of water that it requires. I have only recently discovered what great plants hydrangeas are and have recently planted several but I do feel that I am using a great deal of water in these times of relative drought in our area. But i shall no doubt plant a few more when I get the chance! Thanks, again

Also the oak tree has grown quite a bit as well over the years and its water needs will be quite "large".