I think that is one tree. Maybe browsed to the ground when a small sapling by deer, etc.
I would never remove that large low limb. Weight reduction at the branch tips is way better for the tree plus you never want a large lower limb removed to the trunk as it almost inevitability lead to decay in the main trunk of the tree.
An overall light crown reduction would be good for your oak and reduce leveraged weight and wind loads.
As for thinning inside of the crown near the major limbs, 'Don't do it'.
Make no drastic measures.