Generally, if you have O+ blood, you should be able to donate to someone with A+ blood (O+, B+, and AB+ as well). However, blood from someone of the same blood type is preferred, so the hospital may have just preferred to use blood that matched the patient exactly. There are also certain antigens not accounted for in the typical ABO and Rh blood typing (i.e. the Duffy antigen). They may have checked for a match with one of these and not found it. Or you may have the types backwards, as someone who is type A+ would NOT be able to give blood to someone with type O+. This is because someone with type O has antibodies against the A antigens in type A blood, so if they received a transfusion, their blood would react and agglutinate (clump).