Why is type o blood a universal donor?

Individuals with the blood group O -/+ are called “universal donors” because their red blood cells can be transfused to people of all blood groups. It is also a fact that the Red Cross are searching for O blood donors. I know all of this even though I am only 13 years old cause I am an O blood person myself. I had this question in my head so I researched about it. I hope my answer helps you. Feel free to ask any questions.. :)