Although oxygen frequently limits aquatic ecosystems, broad temporal-spatial oxygen dynamics in streams and consequences for aquatic life are poorly understood. In this perspective, we quantified dissolved oxygen (DO) concentrations (mg·L–1) in six low-gradient streams in Illinois, USA, using semicontinuous data loggers at multiple sites during summer 2005 (a severe drought year) and summer 2006. We then interpreted these patterns in the context of known responses of stream organisms to DO. Daily median and minimum DO concentrations varied among sites, even within streams. Daily mean discharge (m3·s–1) explained as much as 63% of the variance in DO during both the drought and nondrought years, with increasing discharge (often concomitant with declining temperature) causing DO concentrations to increase in most large stream reaches and to decline in many small streams. Biological responses to this variation in DO are uncertain, although our review suggests that minimum daily concentrations within several of these streams may have been insufficient to sustain early life stages of aquatic organisms. We propose a hierarchical conceptual model for understanding DO within temperate streams, suggesting the following constraining factors in descending levels of organization: season, flow (and gradient) plus temperature, geomorphology, organic enrichment, and oxygen demand of macro-organisms.