Why is O- blood okay, but O- plasma not, when blood is mostly plasma?

Whole blood is not given, when you hear someone say "they gave two units of blood", that really means they gave two units of packed red blood cells. In almost no situation that I can think of would whole blood be preferable.

The red cells are then diluted with saline so it will flow in the iv tubing. If they need it administered rapidly, they put a blood pressure cuff around the bag.

O- cannot donate whole blood to anyone. Their plasma contains antibodies that would almost instantly kill a noncompatible recipient. They can donate packed red cells to anyone becaus the most common antigens that cause a transfusion reaction to RBCs are not present. Their plasma, however, is full of them. I have AB+ blood. If I was given whole O- my own cells would not attack it. BUT, the antibodies in the O- plasma would attack my blood cells.

For plasma donation it's the opposite. I have AB+ blood, hence have no antibodies to the A, B, or Rh antigens in my plasma and can be a universal plasma donor.