Answer: Post oak trees are notorious for such decline in residential settings. Anything that stresses them such as construction, soil compaction, herbicide use to the turf beneath their branches, etc. can begin a slow (or rapid) decline.
Lack of water for a tree that is used to receiving regular irrigation can also cause a problem as can overwatering (soggy soil). As a post oak gets older it becomes much less resilient and its ability to recover from stresses or to heal a wound decreases significantly.
There is little you can do to help such a tree. When you have a cold the doctor's advice is to "get plenty of rest and drink lots of fluids". In other words, don't stress your body but put it in a place to heal itself and recover. The same is true for this tree. Give it deep, infrequent (once a week) soakings during summer droughts. Avoid anything that would stress the roots such as soil compaction, soil disturbance, herbicide use on the turf under the tree, etc.
Avoid trying to fertilize the tree back into health. The problem is not lack of nutrition, and this can only compound the problem.
One other possibility is the disease oak wilt. Your description of the leaf sounds a bit like one of the oak wilt symptoms. Contact your County Extension Office for assistance in diagnosing this disease. Dotty Woodson, your Horticulture Agent, can provide instructions for how to go about taking a sample for analysis as well as how to treat the tree if oak wilt is diagnosed.
Good luck with your prize tree.
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