The leaves on my oleander shrub are brown, and the tree does not seem to grow. I purchased it about 9 months ago and it does not have hardly any growth. It is about four feet tall.
Make sure you do not have a pest problem. If there is not move the plant to an area with more sunlight to induce more photosynthesis. Pull away the dead parts of the leaves and make sure they are not getting scorched by the heat. Good Luck and Happy Growing!
Where is the shrub planted, sun or shade, ground or container? Are you feeding, with what and how much? How wet are you keeping the plant? Does the brown color go all the way through the leaves, do all the leaves have brown on them? Does the brown scratch off? What are the minimum and maximum temperatures for this plant? The following article may help answer your question: https://www.gardeningknowhow.com/ornamental/shrubs/oleander/oleander-growing-tips.htm