To my untrained eye, they look like aphids. I think they are not good.

I don't like pestacides (but I do use amdro) I'd cut off the heavily infested parts and spray the lightly infested parts with water until they diminish. PJ

I live in San Antonio. It is my butterfly weed that gets infested with aphids. I find a strong squirt of the water hose takes them right off. They will be back in numbers but you can squirt them again. This allows you to control them without pesticides. They'll eventually die down as their lifecycle comes to an end. And your plants (oleanders are very hardy) will be none the worse.