Abstract Background Dietary antioxidants and obesity are considered significant targets for disease prevention in the elderly. However, a possible cardiometabolic multimorbidity (CMM) correlated to dietary antioxidants and obesity is unknown. This study aimed to examine the relationship between dietary antioxidants and obesity with CMM in the older population. Methods We used data from the NHANES 2003–2018 cycles, including older adults aged 60 and above. Dietary antioxidant status was assessed using the CDAI, calculated from six micronutrients (vitamins A, C, E, selenium, zinc, and carotenoids), and obesity was classified based on BMI. We applied restricted cubic spline models to explore nonlinear associations and logistic regression to assess the associations between pro-oxidant diet, obesity, and CMM. The joint effects of pro-oxidant diet and obesity on CMM were evaluated using additive interaction indices: RERI, AP, and SI, to determine the synergistic impact of these factors. Subgroup analyses by age, sex, ethnicity, and hypertension status were also conducted to assess the synergistic effect of these factors within different population groups. Results A total of 13,178 older adults (mean age 69.85 ± 0.10 years; 45.1% male) were included in this study. A pro-oxidant diet and obesity jointly increased CMM risk, with the Pro-oxidant diet & Obese group having the highest risk (adjusted OR 3.11, 95% CI: 2.39–4.04), indicating that their likelihood of CMM was more than three times higher compared to the reference group (Anti-oxidant diet & Non-Obese group). The Anti-oxidant diet & Obese group (adjusted OR 2.03, 95% CI: 1.59–2.59) and the Pro-oxidant diet & Non-Obese group (adjusted OR 1.33, 95% CI: 1.08–1.64) also showed elevated risks, although to a lesser extent. These findings suggest that both dietary factors and obesity independently contribute to CMM risk, but their combined effect is more pronounced. The interaction between a pro-oxidant diet and obesity was synergistic, with the RERI indicating a positive interaction (0.75, 95% CI: 0.21, 1.29), the AP showing 24% of the combined effect due to their interaction, and the SI indicating a synergistic effect greater than additive (SI 1.55, 95% CI: 1.11–2.16). Subgroup analyses showed stronger interactions in females, younger individuals, non-Hispanic Whites, and those with hypertension. Conclusions Obesity and a pro-oxidative diet are correlated with the occurrence of CMM; there exists an interaction between obesity and a pro-oxidative diet concerning the initiation and advancement of CMM. Subgroup studies revealed more pronounced interactions among females, younger adults, non-Hispanic Whites, and individuals with hypertension.