Which blood group is called universal donor ?

Person with "O" blood group is called universal donor. Why?

Which type of blood group is called universal donor?

Why is person with O-blood group called universal donor?

Which blood group is called as “Universal Donor and ‘Universal Recipient'.

Why is a person having blood group AB called universal recipient and a person with blood group O called a universal donor?

The following blood group is called universal donar

The following blood group is called universal donar

Why is the blood group 'O' called as universal donor?

Why are people with O blood group called as universal donors?