Re: Exporting TBs of data in Oracle schema/database into CSV formatto migrate into PostgreSQL

If downtime is a concern, then, start a transaction on PG, using isolation level - REPEATABLE READ, get the SCN number from Oracle and copy the existing data from each table (table by table) using FDW for the initial data load. And then, if you have Oracle GG license, start the replicat from that SCN so that the data gets to sync eventually. 
And then, when all the tables are in Sync, go ahead and switch your Apps. If we dont have GG license, we need to look for some other extensions or Kafka or something else that could do this job. Remember, migration is always challenging when lots of data is involved, and there never exists a direct magic.