Oxygen and temperature have previously been treated as different environmental stresses and studied separately in most cases. Although the oxygen-temperature interaction may provide new insight into proximate and evolutionary constraints on embryonic development and offspring fitness, it has rarely been studied in oviparous amniotes. We used a two-factor experiment [three oxygen concentrations (12, 22 and 30%) × two temperatures (26.5 and 34 °C)] to identify the effect of the oxygen-temperature interaction on embryonic development and hatchling traits in the Chinese soft-shelled turtle, Pelodiscus sinensis. When eggs were incubated at the critically high temperature, hatching success of turtle eggs was enhanced by hyperoxia but reduced by hypoxia; this result was not observed in eggs incubated at the benign temperature. Hypoxia retarded embryonic development, and reduced body size, locomotor performance and survival rate of hatchings at the critically high temperature. However, the effects of hypoxia were greatly reduced at the benign temperature. Our study demonstrates that oxygen and temperature interact to affect not only the heat tolerance and developmental rate of embryos but also the fitness-related traits of hatchlings, suggesting that interactions among environmental factors impose significant ecological constraints on embryonic development in oviparous amniotes.