Sustainable forage production on the Qinghai-Tibetan Plateau is constrained by limited nitrogen and water resources, which hinders sustainable livestock development. A three-year field experiment was conducted in the northeastern Tibetan Plateau, to evaluate the effects of oat/common vetch mixtures under three nitrogen (N) applications-conventional (CN), half (HN), and none (NN)-on forage yield, root distribution, soil water dynamics, and water productivity (WP). The findings indicated that oat yields in the mixed sowing system were comparable to those in sole oat cultivation, showing a slight increase in 2023. Conversely, common vetch yields in the mixed sowing were notably reduced. Compared to sole oat, mixed sowing increased yields by 7.6%, 44.6%, and 22.6% under CN, HN, and NN conditions, respectively, while crude protein yields saw increments of 32.5%, 142.0%, and 113.6%. In contrast, the mixed system only increased yields by 26.4% and 10.1% under CN and HN conditions compared to sole common vetch. Additionally, mixed sowing resulted in decreased soil water content by 9.5%, 6.2%, and 6.4% under CN, HN, and NN conditions, respectively, in comparison to sole oat cultivation. Oat/common vetch mixtures enhanced root length density in the 20-60 cm soil layer and a strong negative correlation between root length density and soil water content indicated improved root-water complementarity. WP in the mixed system improved under HN and NN conditions, with increases of 45.8% and 12.2%, respectively, while crude protein WP rose by 22.2%, 138.5%, and 93.1% under CN, HN, and NN conditions. Therefore, the mixture in HN achieved the highest dry matter and protein WP, driven by biological nitrogen fixation and balanced nutrient-water interactions. These findings suggest that moderate N input in oat/common vetch mixtures maximizes productivity and resource efficiency, providing an eco-efficient strategy for sustainable forage production and water management in semi-arid alpine regions.