And to follow up what you fully want (to have BGPA or BGPB redistribute a default route into OSPF if and only if they have their EBGP peer sessions established with their respective providers)...
Accept the default route (not just full routes) from your providers if you haven't done so already. I personally always ask for both the default and full routes, so I can do detailed route selection and redistribute the default route as it originates from the providers.
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Network Firefighter
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