Fermented oat kissel is a traditional Russian drink that goes back to the 10th century. It was made by fermenting oats with a piece of rye bread in water for a couple of days, then straining the liquid from the solids, and allowing the starch to settle on the bottom. The starch is the most digestible part of the oat grain, which is what lactic bacteria, and particularly Lactobacillus Plantarum, prefer, and it is where the beneficial by-products of fermentation would concentrate, while bran and germ stay more resilient.
Photo by Beets and Bones. Image of fermented oats in a jar sourced from Beets and Bones. Available at: https://www.beetsandbones.com/wp-content/uploads/2015/04/fermented-oats.jpg