Answer
Te offspring could have "O" type blood if both parents are heterozygous ( IA/i and IBi) . This could result in the offspring inheriting the two recessive genes (i/i).
Work Step by Step
In this system the A allele and the B allele are codominant, but the "i" allele is recessive. If an offspring inherits two recessive genes in this system, his/her blood type will "O" type.