I like shade for mine. I find they grow bigger, more attractive leaves. And the blooms are the same size either way.

If the tree does not have surface roots that could make it compete for water and nutrients, you should be ok. The only trees that I would warn about are Black Walnut and Butternut. Their roots and basically all parts produce/contain juglone in amounts large enough to be toxic to many types of shrubs, including hydrangeas and azaleas.

The landscaper should make sure that the hydrangeas will get appropriate sunlight conditions. You do not say where you are located but, here in Texas, I try to give them shade in the summer months starting around 11am-12pm. Up in the north, you can give them as much sun as full sun condition since the sun is not as harsh in the summer.

But being understory plants in their native SE U.S., they will appreciate afternoon shade no matter where they get it. Enjoy!

Luis

PS - The only warning unrelated to trees is to make sure that the area does not form puddles and that the soil drains well. Oakleafs are notorious for catching root rot if their 'feet' get wet. By the way, which oakleaf hydrangeas you are considering?

In the Seattle area, oakleaf hydrangeas are best in full sun or at least as much sun as possible.
We have had unusual heat for the last week and the oakleaf in full sun is just fine - the Nikko Blues will wilt a bit in the hottest part of the day, but recover nicely at night and appreciate a little extra water.

A friend has "Ruby Slippers" planted in, literally, all-day sun (dawn to dusk) here in zone 5, and they do just fine.
My Mom has a big "Jetstream" oakleaf that is fully shaded (bright shade) in the northern part of zone 5, and it is 6' tall and fully bloomed-out.
It will really depend on the cultivar and your location.

I am in SE Michigan --Zone 6 I guess? I have a call into the landscaper to see what variety of oakleaf hydrangea he is going to use.