Final answer:
Uterine hyperstimulation can be a major complication for clients receiving Oxytocin or Pitocin®.
Step-by-step explanation:
One major complication for a client receiving Oxytocin or Pitocin® is Uterine hyperstimulation. Uterine hyperstimulation occurs when the contractions become too frequent, intense, or prolonged, potentially leading to complications like uterine rupture. This complication can be serious and requires close monitoring by healthcare professionals.