Hi BoroCliff,
What you have to remember is that the A and B antigens are NOT the direct gene products. The direct gene products are, respectively, alpha-1-3-N acetyl-D-galactosaminyl transferase ("A" transferase) and alpha-1-3-N-galactosyl transferase ("B" transferase); in other words, a couple of transferase enzymes.
At birth, these enzymes are not working at their full kinetic capacity, but they are also competing against one another for the Type 2 backbone molecules.
It is quite normal for newborn babies to have weaker A and/or B antigens than do adults.
In this case, mixed-field reactions with the anti-B is almost certainly due to the fact that the "A" transferase was "beating" the "B" transferase in this competition (particularly as it was a newborn - this can also happen, more rarely, in adults).
Although the baby has been transfused with group O red cells, the slight mixed-field reaction in the anti-B means that, although the "B" transferase is "catching up" with the "A" transferase in the "competition", it hasn't quite got there yet!
I think that you would be quite safe in calling this baby group AB.
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