This is because O blood group has both A & B type Antibodies present in it which is essential for A & B Blood group.
Why Blood Group O Is Universal Donor?
It is O-negative blood factors. Yet + blood factors can take - blood factor donationsTherefore, a group O individual can receive blood only from a group O individual, but can donate blood to individuals of any ABO blood group (ie A, B, O or AB). If anyone needs a blood transfusion in a dire emergency, and if the time taken to process the recipient's blood would cause a detrimental delay, O Negative blood can be issued.
Type O is popular because it can be used with any other blood type for like transfusions.