Ask Mr. Smarty Plants

There are a number of possible causes for yellow leaves. Aside from the fact that this has been a very hard year for heat and drought, there are several other things that can cause yellowing. If you just recently planted the oak, it could be suffering from transplant shock,

Yellowish leaves could indicate chlorosis, or lack of iron being taken up by the tree from the soil. This is often caused  by poor drainage and/or dense clay soil, which causes water to stand on the roots. Again, this could  be a problem caused by planting, perhaps without any organic material added to hole, or damage to the tiny rootlets that take up water and trace elements, including iron, from the soil. From a distance, we have no way of diagnosing your tree's problems, nor of recommending a solution. 

To us, the most likely, and simplest answer is that the live oaks are drought deciduous. The live oak is not a true evergreen, but drops all its leaves at once in the Spring, followed quickly by new leaves. Sometimes trees will drop some leaves early to take some of the load off the roots during periods of stress. Don't fertilize any stressed plant. Fertilizer just pushes a plant to put on more leaves, when it is struggling to survive. Unless there is some other problem you didn't mention, we recommend patience. Hopefully, the tree will drop all of it leaves soon, and pop back with fresh new ones.