Oat (Avena sativa L.) is a crop of significant economic and nutritional importance; however, its yield and quality can be substantially reduced by fungal diseases, including powdery mildew caused by Blumeria graminis f. sp. avenae (Bga). The aim of this study was to evaluate the phenotypic resistance of 110 genotypes of Lithuanian oat cultivars to powdery mildew using five pathogen isolates. Based on the results obtained, three genotypes with the highest level of resistance were selected. Among them, one cultivar—Delfin—carries the Pm7 resistance gene, as documented in the literature. The two remaining genotypes were subjected to detailed evaluation using 30 isolates for the purpose of resistance gene postulation, followed by molecular analysis employing STS (Sequence-Tagged Sites) and SCAR (Sequence Characterized Amplified Region) markers, aimed at identifying the presence of the Pm4, Pm5 and Pm7 genes, considered among the most desirable resistance genes against powdery mildew in oat. None of these genes were confirmed in the studied cultivars. The results obtained suggest that the observed resistance may be conferred by other as-yet-unidentified resistance genes. The studied genotypes may therefore represent a novel and valuable source of resistance to powdery mildew, useful in breeding programs aimed at improving disease resistance in oat.