Final answer:
The nurse should first discontinue the oxytocin infusion in response to late decelerations on the fetal heart rate monitor.
Step-by-step explanation:
The nurse should take the action of discontinuing the oxytocin infusion first. Late decelerations on the fetal heart rate monitor indicate a decrease in blood flow to the placenta, which can be caused by uterine hyperstimulation from the oxytocin infusion. Discontinuing the infusion will help alleviate the hyperstimulation and prevent further harm to the baby.