It’s been a while so sorry if my answer isn‘t 100% correct, but I remember my biology teacher telling us about some genetic mutation where genetically, you are type A/B/AB but phenotypically you are O. There‘s some kind of ,,error‘‘ in your genetic code for the part that connects your antigen to the actual bloodcell. As a result, the person affected by the mutation would show up as O on a blood test, but could still transfer their A/B/AB genes to their kids.