It depends on the type of oleanders you have. Spring blooming oleanders should not be pruned any later than the first week in Sept, but could be as early as mid-July or after it's done blooming. so any new growth will harden off before winter. Ever blooming types are a little more forgiving, and can be pruned anytime during the spring - fall. The general rule for pruning most plants is after flowering, if you stick to that, you can't go wrong.