To determine the degree of tolerance to water stress deficit of some oats cultivars and to identify best traits of higher genetic variance related, a field experiment was carried out through two consecutive winter seasons in 2011-2013. That was on the farm of Dept. of Field Crops, Coll. of Agric., Univ. of Baghdad. Three depths of irrigation, 5, 7 and 9 cm (main plots), and three cultivars, Hamel, Pimula and Genzania (sub-plots) were used with RCBD in a split-plot arrangement. The experiment plots were watered 5 times from planting till physiological maturity to give seasonal water of 250, 350 and 450 mm. Genzania cv. gave higher grain yield (5.16 t.ha-1) as average of both years, 121 d to flowering and 151 d to maturity. This cultivar gave 428 racemes.m-2, 30 mg. kernel-1 and higher kernels.m-2 (17800). Crop growth rate increased with increasing water. The depth of 5 cm gave 10.6 while depth of 9 cm gave 12.0 g.m-2.d-1. Irrigating the crop with 7 cm gave best results in raceme.m-2 (419), kernels.m-2 (18500) and grain yield (5.14 t.ha-1). The best combination of cultivars x irrigation depth was with Genzania cv. when irrigated with 7 cm, it gave 464 racemes.m-2 and 5.67 t.ha-1 grain yield. Increasing depth of irrigation from 7 to 9 cm disrupted source/sink relationship, so, harvest index decreased for increased total dry matter. Pimula cv. gave higher dry matter (18 t.ha-1) and lower grain yield. Irrigating with 250, 350 and 450 mm gave 17.6, 15.0 and 10.5 kg/ha grains/mm of water, respectively. Days to flowering and physiological maturity were characterized by higher ratio of genetic to environ-mental variance, and then harvest index and total dry matter. These three traits had 87% or higher h2b.s. However, these traits had similar values of P.C.V% and G.C.V%. It was concluded that these four traits could be used for selection to improve water stress tolerance for this crop, and that water consumptive use of 350 mm.season-1 is the best, and increasing water more than that will increase vegetative growth on the account of reproductive growth.