A 32-year-old woman (O Rh-negative) is admitted to the emergency department with severe postpartum hemorrhage following vaginal delivery. She has received 4 units of O Rh-positive packed red blood cells in the first hour. Her hemoglobin is now 7.2 g/dL, and she remains hemodynamically unstable. The obstetric team is considering further transfusion. Which of the following is the most appropriate next step regarding blood product selection?