This is the expected behaviour on Cisco devices. You cannot redistribute a default route into OSPF. The only way you can have OSPF learn about a default route from another source is to inject it into OSPF using the
default-information originate command. You can use route maps or prefix lists, but the device will simply ignore the redistribution.
Now, if I understand correctly what you want to achieve, then you should be able to do this using the
default-information originate command. Remember that this command will inject a default route into OSPF only if it is in the routing table. Since the default route is being learned via eBGP, it will be in the routing table only when BGP is running correctly.
This means that if the BGP session goes down, the default route learned via eBGP will be removed from the routing table, thus it will not be injected into OSPF.
Now the command
default-information originate always will make the OSPF router advertise itself as the default route even if there is no default route in its routing table. The
always keyword should not be used in your case, to ensure that OSPF will learn of a default route ONLY if a default route exists in the local routing table. More about this particular scenario can be found int he OSPF Default Route section of this lesson:
By default, such an injected route has a default metric of 10, and is a type 2 external route. You can modify this using additional keywords in the command. More about this in the following Cisco command reference:
These references are for IOS devices, but it is largely the same for Nexus devices as well.
I hope this has been helpful!