They are almost impossible to kill, even if you chainsaw them to the ground, so don't worry.

If you want a multiple trunk "tree" oleander, all you have to do is prune out the short stuff from the base, and the side branches, leaving as many long branches as you want. Also remove any dead stubs.

It will take constant, but light, maintenance because they tend to sprout shoots up from the base.

As a landlord, I would be upset with you for butchering landscape plants without permission. Did you talk to your landlord before attempting this?

To Timmy Grover, dont worry about the landlord, it appears you've made an "improvement" by pruning the oleander on the left. Its blooming nicely the others aren't. I live in Texas and have always had oleanders and they "need" to be pruned, trimmed, they will thrive. If you want them as vertical trees prune the smallers shoots, limbs. You are doing a good job !

I agree. :) very nice job indeed, just prune them halfway once the blooms are gone and voila. Oleander 101.