If you have to add or remove offset to timezone, you can use below sql syntax. I will use for Istanbul timezone. Database stamp timezone as UTC and not add Istanbul timezone offset when insert operation. If you have to show on reports as Istanbul time zone, you can use below SQL.
SQL> select to_char( FROM_TZ ( CAST ( datecolumn as timestamp), ‘UTC’) at time zone ‘Europe/Istanbul’), ‘dd.mm.yyyy hh24:mi:ss’) from tbl_test;