I have Red Oak, Scarlet Oak, Post Oak, Live Oak, White Oak and
Hickory Oak. When they are mature they tend to block out sunlight, so I trim them up at least fifteen feet. I have twenty-two "Farmers" that
water and fertilize my Apples, Cherries, Peaches and Almonds. Persimmons and Pawpaw grow wild here and I noticed that they do best on the shady side of the tree. The Persimmons are very cold hardy and
should really be considered. The other naturally occurring understory tree for Oak in the Midwest is Mulberry.
Now, in our forest I've noticed that Wild Raspberry grows abundantly in the dapple shade of the Oaks. I've planted a variety of Raspberry, Blackberry, Boysenberry, Salmonberry and Thimbleberry. All are doing great. I have also found Hawthorn and Elderberry does very well in these guilds.
I'm working hard to build soil quality, so I've planted Goumi, Siberian Peashrub and Comfrey in every guild and I haven't been disappointed. For ground cover, I am partial to strawberries.
I feel like the key is to let the light in. Oaks are really the most adaptive tree in the temperate zone and they make very good use of available
solar energy. Get in there and open them up so that light can find its way through them. I wouldn't worry too much about damage to the tree. I've cut whole trees down, only to see five new trees grow up out of the stump.
One thing to remember about the amazing Oak Tree. That black stuff that gets all over the car, all those flowers that clog up the gutters, and two or three crops of leaves per year is a huge amount of fertilizer and biomass. Don't wast it.