will oleander trees grow back if cut too much
Some gardeners cut trees with a chainsaw at a point above where the branches come out saying it was a hard trim. My concern is that they were not properly trimmed (last week) and they won't come back. The trees were pretty tall, as you can see from some that survived the cutting.
I have never seen Oleander pruned to this severity.
Generally no more than 1/3 of the plant should be pruned away.
You will likely need to wait and see. Best of luck.
Here is a link that explains pruning instructions.