What you need is Ora2Pg, which is a Perl module that is used to export an Oracle database schema to a PostgreSQL-compatible schema. It connects to your database, extracts the structure of the database and generates a SQL script that you can import into your Postgres database. It can dump the actual database schema (tables, views, sequences, indexes, grants), without editing the SQL code generated.
Ora2Pg is also capable of discharging the data in an online transaction, where one can choose what columns can be exported for each table. Features include database schema export (tables, views, sequences, indexes), grants/privileges export by user and group, data export and the SQL query converter. Best of all, the program is free and you can modify it under the same terms as Perl itself. Some more useful documentation on details of the process can be found here.
This was first published in December 2005