OBJECTIVE augmented and not augmented women without dystocia were compared to investigate associations between oxytocin and adverse birth outcomes. Augmented women with and without dystocia were compared, to investigate associations between dystocia and adverse birth outcomes. METHODS a cohort of low-risk nulliparous women originally included in a randomised controlled trial. METHODS the Department of Obstetrics and Gynaecology, Østfold Hospital Trust, Norway. METHODS the study population consists of 747 well defined low-risk women. METHODS incidence of oxytocin augmentation, and associations between dystocia and augmentation, and mode of delivery, transfer of newborns to the intensive care unit, episiotomy and postpartum haemorrhage. RESULTS of all participants 327 (43.8%) were augmented with oxytocin of which 139 (42.5%) did not fulfil the criteria for dystocia. Analyses adjusted for possible confounders found that women without dystocia had an increased risk of instrumental vaginal birth (OR 3.73, CI 1.93-7.21) and episiotomy (OR 2.47, CI 1.38-4.39) if augmented with oxytocin. Augmented women had longer active phase if vaginally delivered and longer labours if delivered by caesarean section if having dystocia. Among women without dystocia, those augmented had higher body mass index, gave birth to heavier babies, had longer labours if vaginally delivered and had epidural analgesia more often compared to women not augmented. CONCLUSIONS in low-risk nulliparous without dystocia, we found an association between the use of oxytocin and an increased risk of instrumental vaginal birth and episiotomy. CONCLUSIONS careful attention should be paid to criteria for labour progression and guidelines for oxytocin augmentation to avoid unnecessary use.