Well this really depends on if the father is homozygous or heterozygous for type A.
But lets say he is homozygous for right now. That mean if he were to produce offspring with a type O women then all of there offspring would be heterozygous for type A.
If the father was heterozygous for type A and he produced with the same type O mother then the offspring would be either type O or heterozygous type A. In this case there is a 50% chance either way.
Hope this makes sense(;