I am thinking to write a converter that takes any oracle query and return Postgresql format of the query assuming table and columns are same.
what I do right now I do timely conversions so I have basic understanding about both and want some expert advice that is it easily possible or not?
Not possible for every query. They each have syntax and functionality that the other does not -- for example, the MODEL clause in Oracle, or PostgreSQL's special form of "SELECT DISTINCT ON".
Mostly, Oracle has functionality that PostgreSQL doesn't: http://docs.oracle.com/cd/B28359_01/server.111/b28286/statements_10002.htm