The default date format is specified in the NLS_DATE_FORMAT database parameter.
You can change the setting by setting this parameter's value init.ora or spfile.ora file.
init.ora or spfile.ora file is read when the database is started.
You can set NLS_DATE_FORMAT using an ALTER SYSTEM command.
You can set the NLS_DATE_FORMAT parameter for your own session using SQL*Plus using ALTER SESSION.
A session is started when you connect to a database and is ended when you disconnect.
The following ALTER SESSION statement sets NLS_DATE_FORMAT to MONTH-DD-YYYY.
ALTER SESSION SET NLS_DATE_FORMAT = 'MONTH-DD-YYYY';