A word of warning - oleander is not fully hardy in this country. I have it outside here in London in a southfacing, very sheltered garden, and it has come through this winter unscathed - but where you live, I wouldn't like to chance it, unless you know its a very sheltered spot.

Bamboo's probably right. If you want to consider alternatives, the rule of thumb for hedges is to plant the bushes 1/2 as far apart as the planned height. For instance: If you plan a hedge 2 m tall, plant tne plants 1 m apart. That will give you the fastest fill-in, without having to pull up every other one, later.