If this is a one-off move then you could extract the Postgres data and use SQL*Loader scripts toput it into the Oracle tables.
If you want to do it over a period of time then you could use the Database Gateway for ODBC (DG4ODBC) to connect to the Postgres database from Oracle using database links and then access the Postgres data.
If you have access to My Oracle Support there is further information in these notes -
Master Note for Oracle Gateway Products (Doc ID 1083703.1)
This has links to further information and the configuration notes for DG4ODBC for various platforms.
This note has alternatives for making a migration -
How To Migrate Non-Oracle Databases For Which a SQL*Developer or Migration Workbench Option Is Not Available (Doc ID 393760.1)