The research material consisted of grains of six genotypes of oats (Avena sativa). The grains of three of them had a brown glume (‘Gniady’, ‘CHD 2875’, ‘CHD 2833’) and the grains of other three genotypes had a yellow one (‘Bohun’, ‘Deresz’, ‘Cwał’). The impact was assessed of their genotype, interactions, orthogonal contrast (oat with yellow glume : oat with brown glume), and vegetation season on the content of amino acids and the quality of proteins in decorticated oat grains. It was proven that the contents of individual amino acids in the decorticated oat grains varied significantly depending on the genotype, vegetation season, interactions between those factors, and colour of the glume. Irrespective of the (Mature Human) or WE (Whole Egg) protein standards, the lysine was an amino acid to limit the quality of the protein in the oat grain. As for the lysine, the value of this limiting amino acid index (CS) was statistically modified by all the factors tested as well as by the orthogonal contrast selected. Values of the CS index of the limited amino acid and the resulting contents of individual amino acids confirmed that the grains with yellow glume were more valuable in terms of the quality of their protein than the grains with brown glume. Based on the profile analysis performed using r_c, a Cohen’s profile similarity coefficient, it was proven that the majority of amino acid profiles of the samples under analyses were highly similar. The amino acid profile of the grain of ‘CHD 2875’ line was characterized by some specific differences; therefore, the oat of this line can be recommended for cultivation as a material containing grains with good quality proteins.