It can also be that one of your parents is O-positive and the other is negative.
For example, if
your mother is
O-positive and
your father is A-negative, than you can be
O-positive,
O-negative, A-positive or A-negative. So you can get the type ( O, A, B, or AB) from one parent and the Rh factor (positive or negative) from the other parent. It is also possible to have the exact same blood type as one of your parents.