A mum with an O baby (blood groups)?

Mum's blood group is an A something.
Unsure of dad's blood group.
Both daughters have the same parents.
Youngest daughter is an A something.
Oldest daughter is an O something.
Is this possible?
When the oldest daughter was born, the mother was given a boy instead of a girl. The mum told the nurses that she had a girl so they swapped the baby with a girl. It was in 1977. So the baby she got given may not be her real baby.
Btw, the mum knew she had given birth to a girl when she got handed a boy. It's because her daughter was put into a room full of other babies pretty much straight away.