Explanation

One parent (at least) must be heterozygous.

Blood group genetics:

Blood type is determined by three genes, A, B, and O. A and B are dominant over O, meaning if someone inherits even one A or B gene, they will have blood type A or B, respectively.

If they inherit two O genes, they will have blood type O. 

Scenario:

If one brother has blood type A (could be AA or AO genotype) and the other has blood type O (OO genotype), they must have inherited an O gene from each parent.

This means that at least one parent must be heterozygous (AO) for the A gene to produce an A blood type child and an O blood type child.