Did anyone try to use EnterpriseDB ?
This database is based on Postgre, and is compatible with Oracle.
It would be intersting to see if that combination works

As far as I know EnterpriseDB is "nearly" compatible, but only on a syntactical level. Meaning Enterprise DB understands the same SQL and PL/SQL dialect as Oracle. As a consequence a developer (!) will have minimal work to port his application from Oracle to Enterprise DB.

But I doubt strongly, that you can connect with an Oracle client via SQL Net to Enterprise Db. Also Enterprise DB will not understand parameter settings etc. intended for Oracle.