Q.Help! I drastically pruned a new oak leaf hydrangea in the fall
Will it ever come back to full growth?

Certified GKH Gardening Expert
It can. The only thing that you can do from here is wait. Sometimes adding a product called B1 from ThriveAlive will help with recovery, but this is not always a sure bet.
From here, just don't do anything drastic like feed it or overwater. It needs to be left alone but only kept slightly moist between drying out thoroughly. This hydrangea needs to be grown is a good amount of shade, and will not tolerate full sun.
This article will give you more information on the care of these: https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/care-for-oakleaf-hydrangea.htm

Thank you for your answer,Bushdoctor. Can't believe I did exactly the wrong thing. I usually research every new plant but not this time.I will hold back from meddling with it further!