Oakleaf hydrangea has a problem
This looks like herbicide damage (phytotoxicity.) Could be glyphosate (commonly in RoundUp and other products), however 2,4-D is very volatile and can float in from anywhere nearby where it was sprayed.
Here more about herbicide damage: https://extension.umd.edu/hgic/topics/herbicide-damage
Watch it to see if it can put out new clean growth. It's possible if this is only affecting a small portion of the shrub, but it may also die if it's extensive. Prune off dead branches.
Last year we had some big privets removed and painted the stumps of the privets with Roundup to keep them from sprouting. We didn't spray any herbicides. The neighbor next to the shrub doesn't spray anything either.
The Oak Leaf Hydrangea is about 20 years old and it is huge. All of the leaves are distorted but they are not dead. The Oak Leaf Hydrangea right next to it is fine. (Please see photo)
I'm wondering if one of the small plants that have come up around it was cut and painted with Roundup.
Do you think there is a chance that it might survive? Does watering it a lot help? I would really like to save this shrub if at all possible.
Thanks for your help.
Given the appearance of the shrub - entirely symptomatic but not the plant next to it - accidental exposure of this one plant to the herbicide is likely. If it has not sent out normal growth by now, it likely will be too weakened or continuing to suffer tissue damage and should be replaced. We understand you'd like to save it, but the lack of normal growth this late after herbicide exposure indicates too much damage has already occurred. Watering more would only serve to stress its root system by preventing enough oxygen from reaching the roots (as happens in plants that are over-watered by other means). The herbicide residue in the environment is likely long gone and broken-down if the application was made last year.