How to advertise a default route into OSPF

Link between IOSV1 and IOSV2 : Link between IOSV2 and IOSV3: Situation: We are running OSPF in our network. IOSV1 is hypothetically connect to an ISP router. Our ISP does not form an OSPF neighborship with us. That means that IOSV2 and IOSV3 can only reach the networks inside the OSPF routing domain. [...]