There is no easy answer to this question as it could be due to any number of things. A photo would aid in my identification but I will give this my best shot.
Your oleander could have a leaf spot fungus. For more information on leaf spot fungus and a prescribed course of treatment, please visit the link below. If it is in fact leaf spot, then you will want to be sure to water the plant at ground level (versus overhead) to keep the foliage dry.
https://www.gardeningknowhow.com/plant-problems/disease/plant-leaf-spots.htm
What do the spots look like - large or small? If they are real tiny then your oleander may have an insect issue such as spider mites or thrips. I would inspect your oleander thoroughly for insects.
https://www.gardeningknowhow.com/plant-problems/pests/insects/spider-mite-treatment.htm
https://www.gardeningknowhow.com/plant-problems/pests/insects/controlling-thrips.htm
The brown-edged leaves could signify the onset of leaf scorch. Here is an article that will provide more information on leaf scorch in oleanders:
http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7480.html
Here is an article that digs even deeper on the reasons for brown edged leaves:
https://www.gardeningknowhow.com/plant-problems/environmental/what-causes-brown-edges-on-leaves-of-plant.htm
