Introduction The Olerud Molander Ankle Score (OMAS) is a widely used validated Patient Reported Outcome Measure (PROM). For clinical research, it is important to determine the Minimal Clinically Important Difference (MCID). The objective of this study was to determine the MCID of the OMAS at several moments in the follow-up, in a cohort of patients that underwent open reduction and internal fixation of unstable ankle fractures with syndesmotic injury. Materials and methods Data for this descriptive study were extracted from a prospective randomized controlled trial, the RODEO trial. The Dutch version of the OMAS was completed at 3, 6 and 12-month follow-up and estimated at baseline. The used anchor-based methods were: mean change and ROC curve. The distribution-based methods were: 0.5SD and minimal detectable change (MDC). Results This cohort included 148 patients. The mean OMAS score in the group with minimal improvement between 3 and 6 months was 15.0 (SD 17.5, 95%CI 9.4–20.6) and between 6 and 12 months 9.5 (SD 17.1, 95% CI 3.1–15.9). The ROC curve between 3 and 6 months resulted in a MCID of 12.5 (AUC 0.72) and between 6 and 12 months, the MCID was 7.5 (AUC 0.78). Using 0.5 SD, the MCID was 10.52 (SD 21.04) at 3 months, 11.37 (SD 22.73) at 6 months and 10.47 (SD 20.94) at 12 months. The MDC was 4.72 at 3 months, 5.20 at 6 months and 4.71 at 12 months. Conclusions The calculated MCID in patients following surgery for unstable ankle fractures ranges from 10.5 to 15.0 at 3–6-month follow-up and from 7.5 to 11.4 at 6–12-month follow-up, depending on moment and method.