Oleander leaf scorch is so widespread here that oleanders are no longer recommended in Lakeway.
The bacterium Xylella fastidiosa clogs water conducting vessels, causing scorched dry leaf tips and margins, yellowing, leaf drop and often death. Early aggressive pruning can sometimes catch the infection before it spreads, but more often than not the bacteria will move rapidly to the roots and then up through the rest of the plant.
Bacterial scorch can resemble heat scorch, so you may want to have your plants tested before taking drastic action. The Texas A&M Plant Disease Diagnostic Lab will test for Xylella for a $50 fee. Read more about Oleander Leaf Scorch in the article Robert "Skip" Richter, Travis County Extension Horticulturist.