Establish genotype/phenotype table
Genotype Phenotype
oo type O
AB type AB
Ao type A
Bo type B
So if parents' phenotypes are O and AB, then we can construct the Punnett square:
O O
A AO AO
B BO BO
So we conclude that the offsprings have phenotypes and proportions
type A (AO) 50%
type B (BO) 50%