These plants are obviously not terribly picky about conditions if they live and thrive in freeway medians. Oleander is drought tolerant once established. They rarely, if ever, need to be fertilized in our local soils or sprayed or pruned. But if you want to prune them to fit into a small yard space or to make them look like a tree rather than a bush, you can do so now. As with all pruning, first cut out any dead or dying branches. Then cut any that are deformed or growing in an undesirable direction. After that, you can prune for size and shape. Cut the plant back to a little smaller than the size you ultimately want, keeping in mind that it will re-grow. Take care not to prune off more than one-third of the plant at a time. Make cuts above nodes that face out in the direction in which you want new growth to go. All parts of the plant are poisonous so do not eat it, burn it, or work on it without gloves. The good news is that this latter quality makes it deer resistant.
More Information: Oleander article, Tulare/Kings County Master Gardeners