What is the chance of two parents with blood types AB and O producing an offspring with blood type O

Zero.

Because person with AB blood group will have (I antigen - antigen on RBCs) has Ia and Ib. While person with blood group O will have only i antigen.

Blood group A: Ia + i; Ia + Ia

Blood group B: Ib + i; Ib + Ib

Blood group O: i + i

Blood group AB: Ia + Ib

Probabilities in the given case:

Ia + i = Blood group A

Ib + i = Blood group B.

Thus the children would either have Blood group A or B. Never O.