Ashok,
There are tools like ora2pg and others which will help you convert oracle-plsql to pg-psql. However, note 2 things:
1. It will not do a 100% conversion.
2. You will need to verify whatever it has converted to ensure that your functionality is not impacted which means testing efforts will be substantial.
All-in-all, as others shared, it is quite a task. Although ora2pg/tools will reduce some work, the time will not reduce because you will still need to verify the conversion and convert yourself whatever is not converted.
HTH