First off, we turn to John Palmer’s How to Brew. In Chapter 12, he addresses Malted Grain - Other Grains and Adjuncts

Oatmeal 1 L Oats are wonderful in a porter or stout. Oatmeal lends a smooth, silky mouthfeel and a creaminess to a stout that must be tasted to be understood. Oats are available whole, steel-cut (i.e. grits), rolled, and flaked. Rolled and flaked oats have had their starches gelatinized (made soluble) by heat and pressure, and are most readily available as "Instant Oatmeal" in the grocery store. Whole oats and "Old Fashioned Rolled Oats" have not had the degree of gelatinization that Instant have had and must be cooked before adding to the mash. "Quick" oatmeal has had a degree of gelatinization but does benefit from being cooked before adding to the mash. Cook according to the directions on the box (but add more water) to ensure that the starches will be fully utilized. Use 0.5-1.5 lb. per 5 gal batch. Oats need to be mashed with barley malt (and its enzymes) for conversion.

So let’s first list out the various forms that oats are available in:

  • Whole
  • Steel-cut
  • Rolled
  • Flaked
If the oats are rolled or flaked, then the starches (what you want in your beer) have been gelatinized and are essentially immediately available.

If the oats are whole or steel-cut, then you need to do a bit of gelatinization yourself. Palmer calls it cooking, and other sites call it a “cereal mash”. All you need to do is cook it in some water for 15 minutes, and have it be the consistency of cereal (not oatmeal). 

Finally, Palmer points out that the oats need to be mashed with other grains. Oats do not have the enzymes that are present in barley, and remember barley is the grain that is sold as “malt”, anything from 2-row pale malt and pilsener malt to black patent malt, and everything between. So, it is not possible to mash or steep the oats separately. If you are doing an extract brew, you can of course simply add some malted barley to your small steeping pot along with the oats, and so in that way provide the enzymes necessary. 

Now let’s move on to my next source, a BYO article from October 1997 titled Oatmeal Stout: Style. Aside from being a great read if you like oatmeal stouts, they detail when you will need to use a temperature rest when using oats.

The thick consistency of an everyday bowl of cooked, breakfast oatmeal results from the high gum content of oats. These gums consist of beta-glucans, which are essentially long chains of many glucose units linked together. The difference between these beta-glucans and starches (which are also chains of glucose molecules) is in the structure of the bonds between the individual units. In well-modified malted grains, beta-glucan levels are low because these bonds are broken down during the germination phase.

In all-grain oatmeal stouts using 5 to 10 percent oats in the grist, the gumminess of the beta-glucans is usually not excessive, and a single-step infusion mash can be used. However, when using higher percents to get greater smoothness and body, you must reckon with the gums to do an effective sparge. This is done by adding a second temperature step in the mash, called a beta-glucanase rest. The enzyme beta-glucanase works best in the temperature range of 104° F to 122° F.

Holding the mash temperature for half an hour in this range before the saccharification rest is sufficient to reduce beta-glucan content significantly.

So when using 5-10% percent oats, you do not need a temperature rest. However, if you go above 10% oats, the gummy stuff (beta-glucans) can interfere with your sparge effectiveness. In plain terms, you might get a stuck sparge or a slow sparge, which could impact your sugar extraction (efficiency). So it’s not really necessary from a chemical standpoint, it’s more about having an effective process. And of course, if you for some reason went up to 30% oats, then yes I fully imagine you would not be able to complete your sparge unless you did the beta-glucan rest, which lowers the amount of gummy stuff in the oats.

Personally, I wouldn’t worry about it until I hit 20% because I’m adventurous like that. Also, if you use an Igloo cooler and would have trouble doing a temperature step on a 10-15lb grain bill, I suggest doing a side mash in a smaller metal pot that can be directly heated. Make sure that you include some amount of malted barley in there in order to incorporate the enzymes that break the sugars down. So for instance if you have 2 lbs of oats, include maybe an equal amount of your base malt. The temperature rest won’t hurt the barley, but as stated will significantly reduce the gummyness of the oats. Do this first, and then start your full mash (60+ mins) and add in the oat/malt mixture. 

To summarize, there are four aspects you need to be aware of when using oats.

Type of oats
Whole, steel-cut, rolled, or flaked

Cereal mash
You need to cook whole or steel-cut oats (not rolled or flaked) for 15 minutes with enough water to have a cereal-like consistency prior to adding to your mash

Temperature (beta-glucan) rest
If you are using more than 10% oats in your grain bill, consider doing a temperature rest between 104f-122f for 30 minutes

Incorporate oats with malt
Oats do not have the necessary enzymes, so no matter what type or how much, always mash the oats with malted barley

I hope this has shed some light on how to best use oats in your beer.