Add 1 cup of oats into a high speed blender or food processor, close the lid and process for 2 minutes or until oat flour forms. If using a blender, pause and shake it once halfway through.
Then transfer ground oats into a medium size fine mesh sieve and sift over a large bowl only until small pieces of whole oats are left in the strainer. Discard them.
Transfer oat flour in an airtight container.
Notes
Store: Store in an airtight container, well sealed bag or Tupperware in a cool dark place like pantry for up to 3 months.
Freeze: Freeze for up to 6 months.
Steel cut oats: For this type of oats, I recommend to use only a high speed blender.