Answer:
There is no problem for two parents having the same blood type to get married, it's just because both of them have the ( O blood type ) all the children will be ( O type ).
The characteristics of ( O type ) in the field of blood transfusion, that the person with this type can receive blood from others only if it is (O), and they can donate blood to any type.
in summery : O type is a general donor, but a limited receiver for it self.