Studies suggest that vitamin D (VitD) may reduce oxidative stress (OS) in multiple sclerosis (MS) patients. This study aimed to compare the effects of various VitD doses on OS in relapsing–remitting MS (RRMS). A 6-month supplementation was introduced using two doses of VitD: 2000 IU/day in the high-dose group (HD, n = 23) and 15,960 IU/month in the low-dose group (LD, n = 29). Significant differences in body weight, height, and age were found between groups. A significant increase in the level of VitD (25(OH)D) was noted in both groups (p < 0.01). A significant increase was observed in the levels of LF and MDA (p < 0.01) and a significant decrease in the concentrations of PSH (p < 0.01), CuZnSOD (p = 0.02), and TOS (p < 0.01). A significant positive correlation was observed between serum VitD and SOD (R = 0.38, p < 0.01) and MnSOD (R = 0.31, p < 0.05), as well as a significant negative correlation between serum VitD and MDA (R = −0.31, p = 0.05) at the beginning of the study. At the end of the study, a significant positive correlation was observed between serum VitD and SOD (R = 0.34, p < 0.05) and CuZnSOD (R = 0.51, p < 0.01). In RRMS patients, the VitD doses are probably insufficient to induce a beneficial effect on the pro- and antioxidant balance.