The study was conducted to determine the effect of three harvest maturity stages on nutritive value of oat hay and milk yield and composition. Total of 9 Holstein Friesian lactating cows with nearly similar lactation stage and body weight were fed on three rations with proportions of 40% oat hay from different maturity stages i.e. boot, flowering and dough and 60% total mixed ration. The results showed that animals fed with oat hay from flowering stage (ration II) had significantly higher (P<0.05) dry matter (DM) intake followed by dough (ration III) and boot stage (ration I). Similarly, intakes of crude protein (CP) and crude fat (CF) were significantly higher (P<0.05) on ration II compared to rations I and III. Neutral Detergent Fibre intake (Kg/day) in rations I, II and III was 5.4, 6.8 and 6.19, respectively. Intake of Acid Detergent Fibre (Kg/day) was higher in ration II while lower in ration I. Acid Detergent Lignin intake (Kg/day) in three rations ranged from 2.26 to 2.61. In vitro DM digestibility (%) significantly increased (P<0.05) in ration II compared to ration I and III. Intake of minerals (g/day) of Calcium and Magnesium did not vary significantly, however intake of Manganese, Copper, Zinc and Ferrous decreased with advancement in oat growth. Milk yield (Kg/day) from rations II was significantly higher (P<0.05) than rations I and III, while milk composition was not significantly affected. The DM yield of oat hay was significantly higher (P<0.05) at dough stage. It was concluded that good quality of oat hay and higher milk production could be attributed to the flowering stage of harvest maturity.