Abstract Anti-oxidant/Pro-oxidant oxidant imbalance leads to chronic inflammation and insulin resistance can lead to the development of metabolic syndrome (MetS). The oxidative balance score (OBS) is a tool for assessing oxidative stress associated with MetS risk. However, the association between OBS and mortality in patients with MetS remains unclear. This study analyzed 10,647 MetS patients from the 1999–2018 National Health and Nutrition Examination Survey (NHANES). OBS were calculated using a combination of 16 dietary and 4 lifestyle factors. Multivariate Cox proportional hazards regression models, Kaplan–Meier survival analysis, restricted cubic splines (RCS), and subgroup analyses were used to evaluate the potential association between OBS and the risk of all-cause and cardiovascular mortality. Sensitivity analyses confirmed the robustness of the results. This study found that OBS was inversely associated with all-cause and cardiovascular mortality in patients with MetS, a result consistent across most subgroups. Both the Kaplan–Meier curve and RCS analysis supported these findings. Sensitivity analysis was used to verify the robustness of the results. Maintaining an antioxidant-based diet and lifestyle may help reduce the risk of all-cause and cardiovascular mortality in patients with MetS. These findings underscore the significance of incorporating antioxidant-rich dietary patterns and behavioral practices in strategies aimed at preventing and managing MetS.