plasma or serum transfusion:when you give blood transfusion, you give rbcs you don't give serum.
universal donor: blood group o persons will not have any antigen on their rbcs so when given to any patient it will not provoke antibodies. However, they have antibodies to a and b in the serum so they cannot accept any group other than blood group o.
universal recipient: blood group ab persons will not have any antibodies in their serum (because they are self tolerant to their own a & b antigens) so they can accept any abo blood group that you transfuse to them. However, you cannot give their blood to other groups as the a and b antigens on the surface of rbc will provoke the antibodies.
Your kaplan question is correct, the serum of blood group o has antibodies to a & b and will certainly agglutinate an ab rbc. However, this is not the case in blood transfusion in which you give (o rbcs) to (ab rbcs and ab serum which has no antibodies) patient.