Blumeria graminis f. sp. avenae (Bga) is the causal agent of powdery mildew in oats (Avena sativa). It is an important foliar disease which can cause yield losses of 10–40%. In this study, we assessed powdery mildew resistance in the commercial oat cultivars; Barra, Binary, Delfin, Elison, Husky Isabel, Keely and Yukon in the field in Ireland and under controlled glasshouse conditions using an Irish Bga isolate. Powdery mildew resistance was consistently observed in Delfin, Elison, Yukon and Binary while Barra was susceptible. The resistance response of each to Bga was further characterised by scoring papillae formation, haustorial encasements and programmed cell death (PCD) using microscopy. Finally, these cultivars were screened with a panel of Bga isolates, used together with oat lines containing known resistance genes (Pm1 and Pm3–Pm10), to assess the breadth of resistance and infer the presence of any Pm genes. This study suggests that Pm1, Pm3 and Pm8 are no longer effective in Ireland. This is likely due, in part, to the previous over-reliance on the cultivar Barra since 1985. While Pm4, Pm5, Pm6, Pm7, Pm9 and Pm10 were found to still confer resistance to an Irish Bga isolate. Using cultivars with different resistance genes could help to slow the erosion of powdery mildew resistance in the field.