A field experiment was conducted over five years to determine the effect of season-long kochia interference on oat yield and quality. Kochia interference did not affect oat height, test weight, 500-kernel weight, or groat percentage. Similarly, ash, starch, and total β-glucan percentages in oat groat were not affected by kochia interference. Oat grain yield was reduced in 1991 and 1994 by 30 kochia plants/m2, the highest density. Lipid and protein content were not affected by kochia except in 1992 where protein content was reduced and lipid content was increased by kochia.