Abstract: Background and Objective: In recent years, consumption of fermented vegetarian milk as a functional food was increased due to its beneficial health and there were many trials to improve the quality properties of fermented vegetarian milk by different modifications. So, the aim of this research was to enhance the functional attributes of fermented oat milk by using probiotic cultures and study the effect of fortifying oat fermented milk with whey protein on the quality of final product. Methodology: Three experimental fermented milk samples (labeled as FCM, FOM and FOM+DWPC) were made from cow milk, oat milk and oat milk fortified with 2% dried whey protein concentrate, respectively. ABT-3 starter culture containing Streptococcus salivarius sub sp. thermophilus, Lactobacillus acidophilus and Bifidobacterium bifidium was used for fermentation. Samples were taken when fresh and during cold storage for analyses. Results: Significant differences were observed in dry matter, fat, total nitrogen, ash, crude fibers, titratable acidity, diacetyl and acetaldehyde contents between fermented cow and oat milk. Fortification the FOM with whey protein caused a significantly increase in the dry mater, total nitrogen, titratable acidity, diacetyl and acetaldehyde contents of the final products. Dynamic viscosity of fermented oat milk had significant higher than that in fermented cow milk. The viability of ABT culture strains in all samples with or without whey protein was higher during the cold storage than the recommended minimum levels (106 CFU mL–1 or g). Fortification fermented oat milk with whey protein had enhanced the viability of ABT culture strains in final product along the cold storage period. All the fermented cow and oat milk samples gave a good total impression. Conclusion: The results of this study indicate that oat milk could be used as good raw material for producing new functional fermented product with high acceptability and a high viability of bio-starter culture along the cold storage. Fortification oat milk with whey protein could be enhancing nutritional values and viability of probiotic strains in the functional fermented oat milk.