The symptoms on the tree are not sufficient to distinguish between the two conditions you have mentioned - drought and oak wilt. Perhaps the rate of symptom development could provide a clue. Since this is a red oak, it is highly susceptible to the oak wilt fungus and would probably only last 6 - 8 weeks after the initial appearance of symptoms. Drought, however, would develop more slowly. This distinction is not totally reliable, so there is a degree of uncertainty associated with the logic. Should you want to be more definitive, I recommend you send a sample in to the Texas Plant Disease Diagnostic Laboratory. On the website of the clinic (http://plantclinic.tamu.edu/), there are instructions on how to collect and submit samples. Another resource for more detailed information can be found at the website http://texasoakwilt.org/. That site has listings of certified arborists in your area that, for a cost, can observe the tree more carefully and perhaps get a more definitive idea of what might be the cause of the symptoms. Either scenario, drought or oak wilt, could be responsible.