Oracle double byte characters sets

If your operating system is enabled for a single-byte character set, it does not automatically accept double-byte character sets. You must enable your database to store monitoring information that comes in double-byte character strings.
So while creating a database includes the CHARACTER SET UTF8 string. This string enables storage of double-byte character sets.

Enabling the database for double bytes character set is just setting the character set to UTF-8.

To make sure the database is UTF-8 enabled execute the below query.
select value from NLS_DATABASE_PARAMETERS where parameter = 'NLS_CHARACTERSET';

result 1: WE8ISO8859P1
result 2: AL32UTF8

If you get UTF8 then it is UTF-8 enabled