Some protocols use
20-40 IU in 1 L normal saline/Ringer's lactate as an IV infusion. Follow your hospital's PPH protocol where available.
WHO PPH guidance notes that 10 IU IV is commonly used initially, followed by 10-20 IU infusion if needed.
PPH is an emergency: simultaneously do uterine massage, assess for retained tissue/trauma/coagulopathy, obtain help, resuscitate, and give tranexamic acid early when indicated. If bleeding continues despite oxytocin, additional uterotonics and escalation are needed.