Cereal grasses, in particular oat grass, are considered to be a potential component of nutritional supplements. The objective of the research study was to determine the basic chemical composition and contents of total polyphenolic compounds in the grass of thirteen oat genotypes. Based on the research analyses performed, it was found that the grass of the oat genotypes analyzed did not differ in the contents of dry mass and fat. The ranges of variability were 93.1 ÷ 96.6 and 2.15 ÷ 3.55, respectively. However, it was reported that the content of protein in grass of various oat genotypes differed and this fact was confirmed statistically with regard to the traditional form: 21.1 % and the naked form: 23.1 %. The content of total polyphenols in dry mass of the oat grass differed statistically significantly, but it was not reflected in the differentiation of antioxidant activity, which ranged from 52.80 to 67.65 % of RSA. The ‘Kasztan’ cultivar was characterized by the highest content of total polyphenols in dry mass (85.40 mg/100 g d.m.), and the MHR-PO-0512 strain was characterized by the lowest: 72.85 mg/100 g d.m. expressed as the chlorogenic acid equivalent. Moreover, the increase was confirmed in the content of total polyphenols in dry mass of the grass along with the increase in the content of dry mass.