A little information on donor compatability:
When it comes to donating blood/blood products compatibility relies specifically on the antibodies in the serum of the recipient. Here is a quick synopsis of what types can donate to which other blood types
Recipient Donor
A A or O
B B or O
AB A, B, AB, or O
O O
Then we bring in the Rhesus factor (named after the Rhesus monkey that this testing was originally performed on) or more commonly called the Rh factor which is either positive or negative.
If you are Rh negative, you can (should) only receive Rh negative blood. If you are Rh positive, you can received Rh positive or negative blood. Through a little deduction you can reason that the best blood type to have and be a recipient is AB Pos. You can receive blood from anyone without out risk of a Hemolytic Transfusion Reaction (HTR). The best blood to have as a donor would be O Neg since your blood type has no antigen (H-substance) to stimulate an antibody response from the recipients blood.
There are exceptions and other limiting factors but that would take too long to get into and not worth the discussion here. In Audrey's case she could accept blood/marrow donations only from O pos and O neg blood types. But with Bone Marrow there are a lot of other low incidence antigens and CcDEe antigens that are present and would need to be tested for. That's why it is not always as simple as blood type compatibility. Also, rest easy in knowing that O pos and A pos are the two most common blood types, so there will be plenty donors to test from. I am A pos and would be no good for Audrey, but I would donate if I could.