Where is this large bed going to be?
Are you going to bury the pin oak roots under soil that you truck in to create this large bed?
Be careful you don't smother them.
I had a very large pin oak tree in front of my house a couple of years ago when I live in PA.
I planted all kinds of plants right under it, but didn't put a whole lot of soil on top of the roots, just enought to grow the plants in.
I planted impatients, begonias, calidums, any small shade loving plant will do fine under them.
Just not something with long roots.
By the way, their roots broke the water line coming into my house, and it cost me thousands of dollars to have a plumber jack hammer the sidewalk and repair the water line.
Their roots are strong and if it doesn't rain for a while, watch out.
I had no idea they would do that, or I would have had it cut down earlier.
Oh well, live and learn.
Your bed is going to look nice, just not too much dirt.

No, I will not be trucking in soil. Just 'spot planting' and amending each hole with mushroom compost and soil conditioner. The hard part is that the back of the bed is so shaded but the front gets such strong sun. Plus the oaks suck up the water. I'm thinking some autumn fern might do well in the back part.