Objective: A decrease of obstetric anal sphincter injuries (OASIS) was observed after preventive interventions were implemented at a Norwegian university hospital. The aim was to investigate whether the improvement had sustained over the following years. Materials and methods: We performed a retrospective cohort study of 18 258 singleton vaginal cephalic births, ≥37 + 0 weeks of gestation during 2012–2017, examining data from the hospital's birth journals and separate registration forms. Interventions to prevent OASIS were implemented in 2011, and training in practical skills was repeated each year. Main outcome measures: The main outcome was OASIS (n = 377). Results: Frequency of OASIS overall decreased from 3.6% prior to 2011 to 2.1% after the intervention and sustained at that level throughout the study period. A trend of fewer OASIS among spontaneous deliveries, decreasing from 2.1% to 1.2% (p = 0.01) was observed, but no trend was seen for instrumental deliveries (p = 0.37), where the incidence fluctuated between 4.0% and 9.3% with an average of 6.5%. Primiparity, increased maternal age and increased fetal head circumference were associated with more OASIS in spontaneous deliveries. In instrumental deliveries, primiparity, occiput posterior position and increased fetal head circumference were associated with more OASIS, whilst episiotomy was associated with fewer OASIS. Conclusion: The incidence of obstetric anal sphincter injuries maintained at a similar level of around 2.1% during the six following years after introducing preventive interventions. Regularly repetition and practical training seemed to be effective.