Oracle has 3 level that one can set character set

one is the database level which is chosen at the instance creation time and cannot be changed second is the instance level character set, which can be changed but it requires a restart. Third is session level, which can be changed while the session is in progressed. One can set a database trigger to change this when a certain user logs in. or change it in the init file.

Session level takes precedence over instance lever and instance level overwrites database level charset.

Lookup commands:

SELECT value$ FROM sys.props$ WHERE name = 'NLS_CHARACTERSET';
SELECT * FROM V$NLS_PARAMETERS WHERE PARAMETER LIKE '%CHARACTERSET%';

database charset:

SELECT * FROM NLS_DATABASE_PARAMETERS WHERE PARAMETER LIKE '%CHARACTERSET%';