Accessions of cultivated oat (Avena sativa L.) from the United States Department of Agriculture-Agricultural Research Service Small Grains Collection in Aberdeen, ID were characterized for adult plant resistance (APR) and seedling resistance to crown rust, caused by Puccinia coronata f. sp. avenae. Initially, 607 oat accessions with diverse geographic origins were evaluated in field tests in Baton Rouge, LA. Of those, 97 accessions were not fully susceptible and were tested in the field in St. Paul, MN against a diverse P. coronata f. sp. avenae population. Thirty-six accessions that had some level of resistance in both field tests and mean coefficients of infection of ≤20 were further evaluated for APR and seedling resistance. Among these, four accessions (PI 193040, PI 194201, PI 237090, and PI 247930) were resistant to eight P. coronata f. sp. avenae races as seedlings. Twenty-nine accessions had resistance to at least one of the P. coronata f. sp. avenae races. Three accessions (CIav 2272, CIav 3390, and PI 285583) were fully susceptible to all eight P. coronata f. sp. avenae races as seedlings. Further evaluation of the three seedling-susceptible accessions at the flag leaf stage in a growth chamber resulted in moderately susceptible to moderately resistant responses. The resistance sources presented here may contain genes not deployed in elite oat varieties, and may be useful for future crown rust resistance breeding. The adult and seedling resistance found in accessions of the cultivated oat species is especially valuable because it avoids problems associated with the transfer of genes from wild species to cultivated oat.