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Irrigated oats is an important forage-crop for livestock industry in arid and semi-arid zones; however, there is a lack of information regarding the relationship of dry matter yield and crop actual evapotranspiration or water demand; then the objective of the study was to develop mathematical models to describe yield and water use efficiency at different levels of irrigation water added. Maximum (P<0.05) yield (12.78 t ha(-1)) was found when 610.13 mm of water was applied, highest (P<0.05) water use efficiency was 2.36 kg of forage m(-3) with 463.64 mm water applied and yielded 11.22 t ha(-1), harvesting at the milky-dough stage (155 d after sowing or 1,291.5 degree days). Crop characteristics and mean effect of the soil evaporation (Kc), in function of the degree days, were better described by a cubic model. Using these models, in addition with the potential evapotranspiration, it is possible to estimate daily water demand, and it can be concluded that 7.15 mm d(-1) is the maximum demand while 5.8 mm d(-1) is the optimal demand without compromising yield. These models are a useful tool to optimize forage production from irrigated oats based on time and amount of water to be applied.