Objective: Obstetric anal sphincter injuries (OASIS) occur in approximately 3-6% of vaginal deliveries and are the leading risk factor for late onset faecal incontinence. We aimed to assess the effect of irritable bowel syndrome (IBS) on severity of faecal incontinence after immediate primary repair of major OASIS (Grade IIIb-IV). Design: Prospective cohort study Setting: Zaans Medisch Centrum, Zaandam, The Netherlands Population: Women who underwent a primary repair of major OASIS over a 2-year period (Group A), a control group consisting of primigravid women (Group B), and another control group who underwent elective Caesarean section (Group C). Methods: Participants were assessed with ultrasonography within 12 weeks, then a follow-up questionnaire after at least 12 months. Main outcome: Wexner faecal incontinence scores and presence of IBS based on Rome IV criteria. Results: There were 211 total patients included, and mean follow-up time was 26 months after sphincter repair. Ultrasonographic sphincter defects were detected in 37% but did not affect faecal incontinence score (p=0.16). Patients with IBS had significantly worse faecal incontinence (p<0.001), and interestingly in women with OASIS those without IBS had comparable symptoms to the control groups. Inability to defer defaecation for 15 minutes was also associated with worse faecal incontinence (p=0.003). Conclusion: After OASIS repair faecal incontinence was significantly worsened by the presence of IBS, or in women with an inability to defer defaecation. Presence of ultrasonographic sphincter defects did not correlate to a clinical difference in faecal incontinence scores. Funding: Nil. Keywords: OASIS, sphincter, irritable bowel syndrome, incontinence