Oat milk is a plant-based milk alternative which is usually listed for its low calcium content. However, chicken eggshell powder (ESP) can be used as cheap, easily available and promising calcium source to fortify oat milk. The present study was conducted to investigate the effect of amount (0.5%, 1%, 1.5% and 2%) and sieve size (38 μm, 50 μm, 75 μm and 90 μm) of chicken ESP on the overall quality characteristics of oat milk. Oat milk samples were characterized for their nutritional composition, physicochemical properties, microbiological quality and sensory properties and results were compared with the values of control oat milk and whole cow’s milk samples. Experimental results showed that addition of ESP with variable amounts and sieve sizes did not significantly affect (p>0.05) the proximate composition and gross energy values of oat milk samples. In terms of mineral composition, contents of calcium and magnesium were significantly increased (p<0.05) by the added ESP with values ranged from 32.24 mg/100 g to130.62 mg/100 g and from 4.45 mg/100 g to 10.81 mg/100 g, respectively, while the iron and zinc contents were not significantly altered. Physicochemical properties, except for the titratable acidity (TA), were considerably influenced (p<0.05) by the added ESP: the viscosity, pH and dry sedimentation values varied between 3.81– 6.83 cp, 5.75–6.82 and 5.80–10.01%; and values for the L*, a*, and b* color components were in the range between 56.02 and 64.71, −0.79 and – 4.39 and 2.29 and 8.12, respectively. With regard to microbiological analysis, addition of ESP had a significant impact on both the total plate count (TPC) and total yeast and mold count (TYMC) of samples. The TPC was found to be between 3.20×103 and 1.52×102 CFU/mL, while the TYMC was between no count and 4.66×102 CFU/mL. Considering the hedonic sensory evaluation, the color, mouthfeel, appearance and overall acceptability of samples were substantially influenced (p<0.05) by the added ESP, while the taste and aroma of samples were not considerably altered (p>0.05). Conclusively, the results of this study highlighted that ESP is an excellent source of dietary calcium which enhanced the calcium profile of oat milk with no discernible impact on its taste and aroma. Key-words: oat; oat milk; eggshell powder; calcium; sieve size.