Although I have never tried with an oakleaf hydrangea, all the other species respond to hard pruning well. If you could provide a photo we could narrow down what type of hydrangea but I am guessing a paniculata. If so, these can be cut back almost to ground level and they will respond well. However, I would wait until it loses its leaves before doing so or wait until late winter/early spring. Now is NOT a good time!!

Yes, if it is a macrophylla (not coned looking blooms) or an oakleaf, pruning now would not be good for 2018 bloom production. Too hot. If it is a paniculata, it would respond well to pruning in the Fall. Dead or dried out stems can be pruned at any time. A picture would help.

Pruning ANY kind of shrub at this time of year is not recommended. Doesn't really matter what kind :-) Pruning stimulates growth if done during the growing season (now) and any new growth will be susceptible to winter cold damage so you want to wait until the plant is dormant (no leaves remaining) before any pruning is undertaken.

It's highly recommended that you post pictures of any unknown variety of plant.

Without one, you won't get accurate advice unless you already know what it is.

"Without one, you won't get accurate advice unless you already know what it is."

That's not precisely true :-) There are many general gardening practices that should be followed regardless of the type of plant. One of which is to avoid pruning any woody deciduous plant after the beginning of August.