Yes, originate default, as type 1, when installed on both routers at each end of the network.
/routing ospf instance
set [ find default=yes ] distribute-default=if-installed-as-type-1
Speak OSPF on all internal routers. Leave them in the default area. Do not originate default from the routers in the middle. What you want will happen by default, based on hop count, if you leave the interface costs at the default all the way through.
You may want to manually adjust the split point in the middle to balance how much traffic goes each direction. Just change interface cost at one hop or another, the ends or the middle, up to you.
For failover, just make sure the default route gets marked invalid when the uplink connection is unusable. There are a lot of ways to do that depending on your uplink situation.