In this study the production performance of lactating dairy cows receiving varying levels of oat silage (OS) and lucerne hay (LH) was determined as well as the maximum inclusion level of OS. Oats was ensiled in the soft dough stage. It was fed to 10 multiparous Jersey cows from 8 weeks post partum in a double 5 x 5 Latin square experimental design. Treatments were OS fed ad libitum together with LH at 0, 2, 4, 6 and 8 kg dry matter (DM) per day and a concentrate mixture containing varying crude protein (CP) levels, i.e. 26, 23, 20 17 and 14% CP ensuring iso-nutritious total diets. Each feeding period consisted of a 2-week adaptation period and a 3-week experimental period. With the exception of protein production, production parameters of cows did not differ (P>0.05) between diets containing varying levels of LH and ad libitum OS. Cows receiving OS as the sole roughage source had lower (P<0.05) total DM-intakes than cows receiving other diets. It is concluded that oat silage can be used successfully as the major forage source for dairy cows although the inclusion of at least 2 kg LH per cow per day in the diet is recommended to ensure a sufficient DM-intake and milk protein production.