You can find Oleanders at most any Nursery in the Austin area but they do not belong here because of the alkaline soil. There is very little information online about the issue with Oleander and the disease Cotton Root Rot due to the high PH soil here. Outside of Texas they just call it Texas Root Rot. It is rare for Oleander to remain healthy in the Austin area for more then 3-5 years when planted in the ground and not in a planter where the soil PH level can be controlled. If you were to ask most any landscaper in Austin to plant Oleander they will usually gladly oblige and often it is out of ignorance. Many landscapers are totally unaware that Oleanders are going to be a bad landscape investment because of the pool longevity.
The Cotton Root Rot is caused by a type of fungal activity in the soil. Texas A&M has more in depth information on their website. The fungal damage often start with the margins of the Oleander leaves turning brown. The Oleanders branches will usually start to die back to the ground and new growth will emerge from the base of the plant in an attempt to survive. As the disease gets worse and saps the plants energy reserved the amount of new growth usually tapers off. Oleanders can often live for a year or more with Cotton Root Rot but they will never recover from it.
If you really want to plant Oleander you would need to excavate out all the native soil down 2-3’ deep and replace it with a acidic soil with a high sphagnum peat moss content. Most of the time it is just not going to be cost effective to plant Oleander and you should consider other plant choices.