A method and system for removing foreign grains from oat grains to establish gluten-free oats, i.e., oat grains having gluten levels below 20 ppm and, more preferably, below 10 ppm, relies upon performing a series of mechanical differentiation operations on a combination of grains, including oats and foreign, or gluten containing, grains. The various operations include aspiration, width grading and length grading steps, as well as a late stage density separating step. The resulting oats can be used to make a wide range of gluten-free oat food products, including cereal and granola products.