Oats contain around 13% protein, but the protein in oats is low in the essential amino acid lysine which is an essential nutrient for all horses, especially pregnant and lactating mares and growing youngsters, and horses in work. Therefore the protein is not considered to be high-quality protein. This is true for ALL cereal grains, not just oats.
This drawback can be rectified by feeding lucerne or lupins, which are all good sources of lysine, and mixes with oats well. This will boost the lysine content of the diet, and improve the overall amino acid profile of the meal.