Whole oats are the healthiest, but any oats are a great treat! Mine devour oats in any form, from oatmeal to a handful of dry oats. I used whole oats that I can get at the feed store for 15$ for a 50lb bag, it lasts since it is treats.
Oats are good and chickens love them, but I would give them the chance to get their regular feed down first. The feed is formulated to be a balanced diet. Our pullets have feed available all the time and we wait until 4 p.m. before giving them snacks, including oats.