The objective of the research study was to determine the content of nutrients and to assess the antioxidant properties of a whole grain, husk, endosperm, and bran in different oat cultivars and varieties. The research material comprised grains of the brown-husked oat of ‘Gniady’ cultivar and of the two varieties: CHD 2875/01, CHD 2833/02, as well as the yellow-husked oat of ‘Bohun’, ‘Deresz’, and ‘Cwal’ cultivars. Using AOAC standard methods of analysis, there were analyzed the contents of: dry mass, protein, fat, total carbohydrates, dietary fibre, and mineral compounds in the form of ash. Also, the content of polyphenols was determined using a Folin-Ciocalteau method. Additionally, the ability to scavenge the ABTS^•+ and DPPH^• free radicals was determined. The whole de-husked grains of brown-husked oat cultivar were characterized by a higher content of protein and, at the same time, by a lower content of total fat and total carbohydrates, as well as by a similar content of fibre and ash compared to the yellow-husked oat cultivars. The endosperm of oat grains with a brown-coloured husk contained more proteins, less fat, and approximately equal amounts of total carbohydrates, fibre, and ash compared to the oat cultivars with yellow-husked grains. The bran of the oat cultivar and varieties with brown-husked grains had a lower amount of protein, a higher amount of fibre, and comparable amounts of other ingredients analysed in comparison to the bright coloured oats cultivars. The bran of oat cultivars with brown-husked grains was characterized by the highest content of polyphenols and by the highest antioxidant activity, while the endosperm – by the lowest amount of polyphenols and by the lowest antioxidant activity. The husk, which is a waste product of oat processing technology, contained significant amounts of dry matter, carbohydrates, dietary fibre, minerals, and polyphenols; it was characterized by a high antioxidant activity.