I've never grown oleander so I have no idea, but did you try Google? You could put in oleander plant diseases, or oleander common growing problems. If there is an agricultural center nearby they might be able to help you with this problem.
I've never grown oleander either, but this looks like a fungus to me. Do you see these white spots on leaves as well? Try to wash them off with dishsoap or strong, black tea. Even if it won't help, it can't hurt, so it's worth a try
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