Place 3 cups of old-fashioned oats, also called rolled oats, into the clean, dry jug of a high-speed blender. You can also use a food processor, but the process will take a bit longer to form oat flour.
Blend on high speed, speed 6-7 of a Vitamix, for about 30 seconds. Stop, check the consistency. If it is coarse, keep blending to a maximum of 1 minute. It should form a fine flour, as seen in the picture above.
For food processors, blend up to 1 minute 30 seconds on high speed.
Store homemade oat flour in a sealed jar, in a cooled, dry, and dark place like the pantry, or the fridge. Place the oat flour in an airtight container. It can be plastic, metal, or glass. It stores well for up to 6 months and can be frozen and thawed the day before use.