We recently done the similar migration for one of our customer. We used all opensource tools to achieve this migration process.
We used Pentaho Data Integration tool for doing Online DB migration, which took minimal downtime with CDC{Change Data Capture} approach. Also, we used Ora2Pg tool to migrate the DB objects with some manual syntax modifications.
thats the easy part.
now what about the massive code base of pl/sql and triggers he mentioned ?
Yeah, we need to rewrite the business logic if there are any un-supported features like autonomous transactions, packages, nested procedures, e.t.c.