Perennial grasses' adaptation response to climate change conditions in arid lands is limited. To understand the adaptive behavior of Oloptum miliaceum (L.) R & ouml;ser & Hamasha to survive long drought stress periods in arid regions, we analyzed its ecophysiological responses under water deficit conditions. O. miliaceum plants were subjected to different irrigation treatments to simulate levels of water stress ranging from 250 mm/year (no deficit) to 80 mm/year (severe deficit). From June to November, morphological and physiological parameters were assessed every 15 days. Results showed that O. miliaceum adjusted gas exchange through stomatal closure under higher stress, which affected plant photosynthesis, and growth traits (Number of tillers and leaves per plant). A severe water deficit of 80 mm/year minimized the water loss through stomatal closure, which subsequently reduced photosynthesis and leaf growth, reflecting the water conservation strategies of the plant. The analysis of tolerance indices identified traits associated with improved water deficit stress tolerance, demonstrating a high tolerance level for both predawn and midday water potential and low tolerance of photosynthesis and transpiration (E). In conclusion, O. miliaceum showed opportunistic adaptive strategies under stress with a sensitivity of photosynthesis and growth. Actually, the soil water content constituted the main controller of water potential and physiological response. These findings contribute to understanding how the plant can enhance resilience under extreme drought events in arid lands.