Live Oak Bark falling
It's very hard to come up with a definite answer and solution with just one image of the trunk.
The best thing to do is to contact a Certified, Bonded and Insured Arborist from the San Antonio Arborist Association such as Bartlett Tree Experts to do a complete site and tree evaluation.
As we all know trees are the biggest investment in our landscapes.
It could be a disease, physical damage or the bark is simply "sluffing off" with all the past years spring rains.
For sure from this image, the Arborist needs to expose the flare roots off the crown.
David Rodriguez -Extension Horticulturist
Texas A&M AgriLife Extension Service
Bexar County/San Antonio