Introduction and Hypothesis <p>Obstetric anal sphincter injuries (OASIS) are serious complications of vaginal birth. The burden is expected to be high in Nigeria due to high fertility and aversion to caesarean section (C/S); however, the incidence, effects on urinary incontinence (UI)/anal incontinence (AI), and quality of life (QoL) are largely unknown. This study determined the incidence of and risk factors for OASIS, as well as the effects of OASIS on UI/AI after childbirth, and QoL.</p> Methods <p>A prospective cohort study of post-partum women in Nigeria over a 16-month period, from January 2022 to April 2023. The study consisted of three groups—the OASIS group (group A) and two comparison groups (groups B and C). Group A had OASIS, group B had episiotomy/2nd degree spontaneous perineal tear (SPT), while group C had intact perineum/1st degree SPT. The women were followed-up for 3 months to determine incidence of and risk factors for OASIS, as well as the effects of OASIS on UI/AI, and QoL using validated questionnaires.</p> Results <p>There were 3027 vaginal births with 60 OASIS (1.98%). Incidence of UI/AI was higher with group A than B (<i>p</i> = 0.026 and 0.005, respectively) and C (<i>p</i> = 0.011 and 0.003, respectively). There were no differences between groups B and C (<i>p</i> &gt; 0.05). Risk factors for OASIS were age ≥ 35&#xa0;years, low social class, nulliparity, previous C/S, prolonged labor, and macrosomia. QoL mean scores were lower in group A than group B (<i>p</i> = 0.027) and group C (<i>p</i> = 0.009).</p> Conclusions <p>Approximately two in every 100 Nigerian women experience OASIS following vaginal birth in a hospital setting, which is associated with increased risks of UI, AI, and reduced QoL, underscoring the need for preventive strategies and appropriate postpartum care.</p>