According to ABO system, Persons with :-

                   Blood Group                        Have Antigen
                          A                                        A                                                                                  B                                        B                                                                               AB                               Both A & B                                                                          O                                  No Antigen                                                 
Different antigens have different antibodies that react with each other which can risk the life of the patient but same antigens don't react with each other.O has no antigen so no one has problem with it. So A will allow A & O, B will allow B & O, AB has both so it will allow A & B & O. But O only needs O because A has antigen A and B has antigen B. That is why we say that O is universal donor and AB is universal acceptor. O can give blood to anyone and AB can accept blood from anyone.But O will only take blood with O.
Hope u understand the concept.....
O group blood dont have any antigens henve it is called as universal donor group whereas a group have a antigens and b antibodies and b group have b antigens and a antibodies so they can't donate blood to ogroup peoplw