Oracle Login Context Information

Find the Database instance you are logged into
SELECT sys_context('USERENV', 'DB_NAME') FROM dual;
SELECT sys_context('USERENV', 'INSTANCE_NAME') FROM dual;

Find the operating system user hosting the Oracle session
SELECT sys_context('USERENV', 'OS_USER') FROM dual;

Find the Oracle user currently logged into
SELECT sys_context('USERENV', 'SESSION_USER') FROM dual;

http://www.psoug.org/reference/sys_context.html