From what I’ve seen, the majority of the ancient oaks spawn right on the edge of where the rocky mountains start and the green plateaus. I decided to mine the area around the one near my starting area so I could make a park for my Hearthlings. I was able to mine everything around the oak except for one row/column of additional rock that intersected the tree. As you can see in the following picture, I’ve tried to select this portion of the rock for mining and although most of what was surrounding the tree was removed, this last bit was not.
I think the only difference between this part and the rest of the rock that intersected was that it intersected the roots.
It’s a small thing, but something to be aware of.