There's no automated tool from Oracle that can assist you with the migration.
You can either export all data from your Progress database into flat files containing just ASCII data and then use SQL*Loader to load the content into the Oracle database. Another common way is to configure the Database Gateway for ODBC which allows you to connect from an Oracle database to a foreign database using a 3rd party ODBC driver and you can then use Pl/SQL or even SQL*Plus copy command to copy the data from your Progress database into Oracle. A 3rd option would be to use JAVA by for example loading the JDBC driver for the Postgres database into the Oracle JVM and then by writing some code that opens the connection and fetches data from the Postgres database into Oracle.
More details can be found in the note Options for Connecting to Foreign Data Stores and Non-Oracle Databases - For Example - DB2, SQL*Server, Sybase, Informix, Teradata, MySQL (Doc ID 233876.1) which is available through My Oracle Support portal.
I can't agree there is no tool for the automate performance of this task. I'm representing the vendor company of such a product SQLWays. You can go to ispirer.com to have more information about it. If you provide me with the data volume I'll be able to give you a quote.