Type O blood can generally be donated to anyone with no ill-effects.
However, people with type O blood have lower levels of Von Willebrand factor, a blood clotting agent that may help prevent life-threatening bleeding.
Dr Takayama said the results raised questions about the emergency transfusion of type O red blood cells to severe trauma patients – victims of injuries with the potential to cause long-term disability or death.
All the study participants had suffered severe trauma and been admitted to critical care medical centres in Japan between 2013 and 2016.
The research is reported in the journal Critical Care