Character set conversion

Based on this

db1>SELECT value FROM nls_database_parameters WHERE parameter='NLS_CHARACTERSET';

VALUE
----------------------------------------------------------------
WE8MSWIN1252

To return in the national character set of the database use UNISTR function:

db1>SELECT dump(UNISTR('\00E4'), 1016), UNISTR('\00E4') FROM DUAL;

DUMP(UNISTR('\00E4'),1016)               U
---------------------------------------- -
Typ=1 Len=2 CharacterSet=AL16UTF16: 0,e4 ä

To return in the database character set use TO_CHAR:

db1>SELECT DUMP(TO_CHAR(UNISTR('\00E4')), 1016), TO_CHAR(UNISTR('\00E4')) FROM DUAL;

DUMP(TO_CHAR(UNISTR('\00E4')),1016)       T
----------------------------------------- -
Typ=1 Len=1 CharacterSet=WE8MSWIN1252: e4 ä

If characters are not shown correctly in SQL*Plus use chcp and NLS_LANG as explained here

db1>host chcp
Active code page: 1252

db1>host echo %NLS_LANG%
.WE8MSWIN1252