During 120 years of Polish breeding of oats (Avena sativa L.), dozens of new varieties have been developed. This study was undertaken to investigate the diversity and population structure of 72 Polish oat cultivars released since 1893. The analysis was based on pedigree data as well as ISSR and REMAP marker polymorphisms. The ancestry of common oat cultivars was traced back to 124 cultivars, breeding lines, and landraces. The five most common progenitors were ‘Markische Land-sorte’, ‘selection from Ligowo oat’, ‘Fransk Svarthavre’, ‘Blanche de Siberie’, and ‘selection from Schleswig-Holstein landrace’. We found that at least one of them was present in 78% of analysed objects. The studied cultivars were assigned to four groups according to the period of their breeding (before 1945, 1945–1969, 1970–2000, and after 2000) and six groups according to the breeding com- pany (Strzelce Plant Breeding Company, DANKO Plant Breeding, Station of Plant Breeding in Rogaczewo, Małopolska Plant Breeding Company, Station of Plant Breeding in Borów, and other). A decrease in observed heterozygosity within the groups was observed only in the postwar period (1945–1969). As a result of breeders' efforts and extensive crosses with foreign materials initiated in 1970 and 1980, new alleles were provided to the oat gene pool. The highest number of new varieties came from the Strzelce and DANKO breeding companies. There were no significant differences between modern cultivars derived from different breeding companies. However, very early breeding centres functioning before 1945 had significantly different materials from the modern ones. The population genetic structure of the studied group of cultivars appeared to be quite simple. It was shown that their genetic makeup consisted of two or three distinct gene pools, depending on the method of polymorphism assessment. The performed research proved that Polish oat breeding using traditional breeding methods—such as selection or intraspecific and interspecific crosses—although focused on improving yield and tolerance to biotic and abiotic stress, did not significantly narrow the oat gene pool and has been releasing cultivars that are competitive in the European market.