They will need grit with this.
Oatmeal is whole oats that has been de-hulled and rolled between steel or granite rolers to flatten in. Steel cut is de-hulled and just cut by spinning knives. No difference in nutrient value. Usually more expensive because of the processing. Get a sack of plain whole oats. Cheaper and the birds will not care one way or another.
You can buy steel-cut oats super cheap from the bulk food bins at many grocery stores. I know this because I bought a pound several months ago, then discovered that I didn't care for it! (Too chewy, too hard to make relative to regular oats.)
I've been doling it out in occasional handfuls to my chickens ever since. They seem to appreciate it!