Introduction: Oxytocin augmentation is a common intervention in modern labour care, used to reduce labour dystocia and caesarean section rates. New definitions on the start of active labour, as well as a lack of knowledge on the association between cumulative dose, plasma concentrations, and labour outcomes warranted this research, along with the need to understand women’s experiences of decision-making regarding labour interventions. Methods: Studies I and II were cohort studies, associating the cervical dilation at the time of oxytocin initiation and the cumulative oxytocin dose in primiparous women experiencing spontaneous onset of labour, with adverse labour and postpartum outcomes using logistic regression methods. In Study III, plasma oxytocin concentrations were serially measured during ongoing oxytocin augmentation and thereafter analysed using tandem mass spectrometry (MS/MS). The median and maximum oxytocin concentrations were examined in relation to labour outcomes, and the correlation between the maximum concentration and the number and total duration of contractions was calculated. In Study IV, women with spontaneous onset of labour were interviewed on their experiences of decision-making regarding labour interventions. The transcribed interviews were analysed using reflexive thematic analysis. Results: Oxytocin augmentation initiated ≤5cm cervical dilation was more often associated with caesarean section (p<0.001) and a negative birth experience (p 0.006), but logistic regression indicated no increased risk of operative births (1.28 (aOR 0.78-2.08)). A high cumulative oxytocin dose (>75th percentile, >4370mU/437mL) increased the risk of postpartum haemorrhage (2.77 (aOR 1.77-4.37)), an overdistended bladder postpartum (2.17 (aOR 1.08-4.38)) and a negative birth experience (1.65 (aOR 1.11-2.46)). The median and maximum plasma oxytocin concentration during labour with oxytocin infusion was higher among women with an operative birth (caesarean section or vacuum extraction) compared to women with a spontaneous vaginal birth (p 0.03 and p 0.027), without any differences in remaining outcomes. The experiences of decision-making regarding labour interventions were highly individual. The themes To be a crew member, Choosing to be a passenger and To be left behind were developed, reflecting the varying experiences of labouring women. Conclusions: According to new recommendations on the start of active labour, early initiation of oxytocin should be avoided, if possible, as it may have negative consequences on labour outcomes and women’s birth experiences. Initiating oxytocin infusion in early labour should perhaps be viewed as a form of labour induction and discussed accordingly among healthcare professionals and with women in labour. Healthcare professionals should also be ready to prevent and manage postpartum haemorrhages when high cumulative oxytocin doses have been administered and should use active measures to detect and avoid an overdistended bladder in the early postpartum period. The plasma oxytocin concentration in labour is highly individual, but women with an operative birth had higher concentrations than women with spontaneous vaginal births. The association needs further research to better understand the effects. Labouring women’s need for inclusion in decision-making varies depending on individual, situational and contextual factors; however, most women want to receive information on their labour process and the welfare of their child. By adopting a women-centred approach, offering choice and control in labour and birth, and empowering women through sharing evidence-based information and shared decision-making, women’s satisfaction with decision-making during childbirth might increase, thereby also improving their overall birth experience.