oracle convert DD-MON-YY to DD/MM/YYYY

If you don't provide the NLS_DATE_LANGUAGE parameter, your own session's parameter will be used.

You can override that like so:

select TO_CHAR(TO_DATE('01-JAN-16','DD-MON-YY', 'NLS_DATE_LANGUAGE = English'),
           'DD/MM/YYYY') from dual;

This will affect only this query, nothing else. If you need to work with many dates like this,

ALTER SESSION SET NLS_DATE_LANGUAGE='ENGLISH'

- then you can change it back later, or it will reset to Turkish when this session ends and you start another session.

If you need this change to be made (almost) permanent, put it in your settings in SQL Developer or Toad, or the login.sql for SQL*Plus.