No it can't. It's tricky, but I'll try and explain. Everyone's red blood cells have antigens, and antibodies. Type A has antigen A and antibody anti-B. Type B has antigen B and antibody anti-A, Type AB has both antigens but no antibodys, where as type O has no antigens but both antibodys.
Basically, the same types of antigens and antibodys cannot mix, or else the blood will clot. So for example in your case, A given to O would mean that antibody anti-B would be mixed with the antigen B which is present in O (since O has both antigens), so the blood would clot and your friend would die. O is known as the universal donor, since it has no antigens to mix with the other blood types antibodies so it can be given to all patients of any blood type. However this also means that blood group O people can only receive blood transfusions from other group O people, because any antigens would cause the blood to clot (as it has both antibodys). Understand?
The positive part of the blood group indicates the presence of another antibody called anti-D, but since you and your friend would both have the antibody this wouldn't make a difference. If one of you were negative however, there would be the same problem with your antigens and antibodys mixing.
Basically, the same types of antigens and antibodys cannot mix, or else the blood will clot. So for example in your case, A given to O would mean that antibody anti-B would be mixed with the antigen B which is present in O (since O has both antigens), so the blood would clot and your friend would die. O is known as the universal donor, since it has no antigens to mix with the other blood types antibodies so it can be given to all patients of any blood type. However this also means that blood group O people can only receive blood transfusions from other group O people, because any antigens would cause the blood to clot (as it has both antibodys). Understand?
The positive part of the blood group indicates the presence of another antibody called anti-D, but since you and your friend would both have the antibody this wouldn't make a difference. If one of you were negative however, there would be the same problem with your antigens and antibodys mixing.
