Why there might still be problems with a type O blood donation to a type B blood recipient?

even if the type B was Rh Negative you can give O Positive blood one time in an extreme emergency. Rhogam is also administered at the same time to attempt to prevent the creation of a Rh antibody. but the patient won't have an existing antibody until after being given the positive blood.
if the B is Rh negative and has previously created an Anti-D antibody and the O is Rh Positive then you would have major problems.

if the B has been previously exposed to blood or blood products and created an antibody the problems won't be from the O type but from other antigens that are located on the red cells. There are 34+ that the laboratory screens for when doing a routine crossmatch.

if the B has never been exposed to other blood or blood products it could be a medication that the patient is taking. several medications cause transfusion problems. also some medical conditions such as multiple myeloma cause transfusion problems as well.

This might because of the Rh factor of the red blood cell, whole blood and the plasma. If you're having the red blood cell of O+, you can only donate it to the person who are having the red blood cell of O+, B+ and A+ without any reactions to the recipient. If you have the red blood cell of O-, you can donate it to them who have the red blood cell of O+, O-, A+, A-, B+, B- and AB- any reactions to the recipient.

there are other factors in blood besides the ABO factors, such as the Rh factor etc. Think about other factors besides the ABO that migh cause a reaction in a blood recipient.

If the donor is Rh + and the recipient is Rh -, there would be a reaction.

positive or negative.........