The date value is stored in the database in a binary (not textual) format. Format you used in the to_date() function is not stored, it is not part of the Oracle's DATE datatype. Therefore, when tools like Sqlplus need to print out a DATE value, they need to convert it to text. These conversions are guided by your current NLS (National Language Support) settings.
You can change these settings by altering corresponding session variables - in this case, the NLS_DATE_FORMAT variable, eg:
However, this is only useful in tools like Sqlplus. Do not ever rely on implicit conversions in production code (regardless of the language), as in that case your application gets easily broken as soon as the NLS settings changes.