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Oat ( Avena sativa L.) yield and quality on the northern Great Plains are consistently reduced by frequent drought and wild oat ( Avena fatua L.) competition. Wild oat cannot be selectively removed from oat with herbicides. Identifying genotypes or seed size(s) with high germination potential under moisture stress may facilitate improved seedling vigor, stand establishment, and crop competitiveness. Therefore, a germination study was conducted to determine the effects of genotype and seed size on the germination of oat seed subjected to moisture stress. Large, medium, and small seeds of six common western Canadian oat genotypes were germinated in polyethylene glycol (PEG 8000) solutions with initial osmotic potentials ranging from 0 to −0.4 MPa at 5°C. Generalized linear mixed models fit to the data provided a statistically valid, appropriate, and convenient method to analyze germination data. In all genotypes examined, decreasing seed size and osmotic potential increased median germination time (MGT) and lowered final germination percentage (FGP). Among genotypes, CDC Bell had the fastest MGT while AC Mustang had the highest FGP. Delays in MGT and reductions in FGP resulting from increased moisture stress were similar to those observed in other cereals, suggesting that oat may be as capable of germinating under low spring soil moisture conditions as wheat and barley. The results of this study also indicate that large seed of genotypes such as AC Mustang and CDC Bell appear better suited to germinate under the range of osmotic potentials in this study. Although differences in MGT and FGP between seed sizes in this study were statistically significant, they were generally small and thus, oat germination characteristics may not be substantially improved by screening out small seed from farm‐saved seed.