Hi Sandy,

Thank you so much for reaching out to the UNH Extension Infoline regarding your concerns about your oleander plant. Without seeing a picture of the actual plant, it is difficult to know what the problem is. 

Some potential issues could be over watering, excessive fertilizer, drought stress or the plant may be root bound.

The link below discusses oleander and some potential pests ( which it seems is not the issue with your plant) as well as some fungal diseases which tend to cause spotting on the leaves.  Armillaria is a fungal disease that could cause the leaves to die however, it is unlikely to be the case if sterile potting soil was used when you planted the oleander. There is oleander leaf scorch which is a bacterial disease, most often seen in warmer climates where oleander grows outside all year. Below is also a link about oleander scorch disease.

https://plantclinic.tamu.edu/calendar2019/oleanderxylella/

https://plants.ces.ncsu.edu/plants/nerium-oleander/

I think that a good first step would be to repot the oleander in sterile potting soil and see if that helps.

I hope that this information is helpful. Please feel free to reach out if you have any further questions.