Oracle to PostgreSQL
We also found existing tools tricky to work with, so ended up hacking together a solution using the sqlalchemy library in python. Our general approach was to mirror the source database; hackily convert data types where necessary; create the target database; copy across the data in chunks. 

For an example, see: https://github.com/MIT-LCP/oracle2postgres/blob/master/migration.ipynb. Using this approach, we migrated a fairly large database (~100 schema, with >1000 tables, some tables with >200 million rows) from Oracle to Postgres.

For more detail, see: https://github.com/MIT-LCP/oracle2postgres. The package can be installed with pip install oracle2postgres. If you hit problems when reusing the code, feel free to raise an issue and we'll do our best to help.