Objective: Does an ‘after hours’ delivery affects the rate of third and fourth degree tears (Obstetric Anal Sphincter Injuries, OASIS) Design: a single -centre retrospective cohort study Setting: A major regional Australian hospital Population: All cephalic vaginal deliveries at 37+ weeks over a 3-year period. Exclusions: previous/planned Caesarean section, prematurity, multiple pregnancies, non-cephalic presentations and models of care other than mainstream consultant-led care. Methods: Data collected was age, parity, gestation, body mass index, induction of labour, use of oxytocin, second stage duration, mode of delivery, perineal trauma, episiotomy and birth weight. Main Outcome Measures: Compare the rates and risk factors of OASIS between deliveries occurring “in hours” (8am-8pm weekdays) and “after hours” (8pm-8am, weekends and public holidays) Results: 2404 patients were included. 57.4% of deliveries occurred ‘after hours’. The overall rate of OASIS was 3.12% with no statistical difference between ‘in hours (3.22%) and ‘out of hours’ (3.04%), p=0.8. More primigravid deliveries and spontaneous onset of labours occurred ‘out of hours’ (both p<0.001). More multigravid deliveries, labours requiring oxytocin and prolonged second stage occurred ‘in hours’(p=<0.01). No statistical significance was noted for epidural analgesia (34.0% vs 34.3%, P=0.85), spontaneous vaginal birth (90.4% vs 90.1%, P=0.81), operative vaginal birth (9.6% vs 9.9%, P=0.81) and episiotomy use (17.2% vs 18.4%, P=0.41). Conclusions: No statically significant difference was noted in the rate of OASIS ‘in hours’ and ‘out of hours’ in our centre.